I am experiencing the same issue and have not found a fix yet. I have noticed that the disconnect sometimes occurs after I shut down one of my wireless network devices (a Toshiba laptop)....this could be pure coincidence but has happened a few times already. The ADSL2 line disconnects sometimes after my laptop goes to sleep. Regardless, this fault/bug should not be happening. I have only had this modem/router for about 10 days.
To get the connection back up I have to power cycle the router (clicking on 'connect' under Status > Modem Router does not appear to work (even when waiting for 10 minutes after clicking connect). When the ADSL2 connection is up it appears to stay up until extended periods of network inactivity. I have checked my ADSL settings and they appear to be ok. The PPPoE connection settings are also set to "keep alive"....this is clearly not happening. I have tried changing the redial period from the default of 30 seconds to 60 seconds but so far this has shown no improvement.
I am currently experimenting with different settings for the ADSL & wireless to search for improvements to this problem. Here are some of the system log messages I received:-
Sat, 2011-04-09 11:50:41 - ADSL is disconnected.
Sat, 2011-04-09 11:50:41 - WAN will be stopped.
Sat, 2011-04-09 11:50:43 - Terminating on signal 15.
Sat, 2011-04-09 11:50:43 - Enter: tdb_store.
Sat, 2011-04-09 11:50:43 - tdb_store: calling tdb_update.
Sat, 2011-04-09 11:50:43 - Enter: tdb_update.
Sat, 2011-04-09 11:50:43 - Enter: tdb_store.
Sat, 2011-04-09 11:50:43 - tdb_store: calling tdb_update.
Sat, 2011-04-09 11:50:43 - Enter: tdb_update.
Sat, 2011-04-09 11:50:43 - Enter: tdb_store.
Sat, 2011-04-09 11:50:43 - tdb_store: calling tdb_update.
Sat, 2011-04-09 11:50:43 - Enter: tdb_update.
Sat, 2011-04-09 11:50:43 - Couldn't increase MTU to 1500.
Sat, 2011-04-09 11:50:43 - Couldn't increase MRU to 1500
Sat, 2011-04-09 11:50:43 - LCP down.
Sat, 2011-04-09 11:50:43 - Connection terminated.
Sat, 2011-04-09 11:50:43 - Connect time 847.5 minutes.
Sat, 2011-04-09 11:50:43 - Sent 43066594 bytes, received 606289861 bytes.
Sat, 2011-04-09 11:50:43 - Enter: tdb_store.
Sat, 2011-04-09 11:50:43 - tdb_store: calling tdb_update.
Sat, 2011-04-09 11:50:43 - Enter: tdb_update.
Sat, 2011-04-09 11:50:43 - Doing disconnect
Sat, 2011-04-09 11:50:43 - Exit.
Sat, 2011-04-09 11:50:43 - Exit.
Fri, 2011-04-08 12:22:23 - ADSL is disconnected.
Fri, 2011-04-08 12:22:23 - WAN will be stopped.
Fri, 2011-04-08 12:22:24 - Enter: tdb_store.
Fri, 2011-04-08 12:22:24 - tdb_store: calling tdb_update.
Fri, 2011-04-08 12:22:24 - Enter: tdb_update.
Fri, 2011-04-08 12:22:24 - Enter: tdb_store.
Fri, 2011-04-08 12:22:24 - tdb_store: calling tdb_update.
Fri, 2011-04-08 12:22:24 - Enter: tdb_update.
Fri, 2011-04-08 12:22:24 - Enter: tdb_store.
Fri, 2011-04-08 12:22:24 - tdb_store: calling tdb_update.
Fri, 2011-04-08 12:22:24 - Enter: tdb_update.
Fri, 2011-04-08 12:22:24 - Couldn't increase MTU to 1500.
Fri, 2011-04-08 12:22:24 - Couldn't increase MRU to 1500
Fri, 2011-04-08 12:22:24 - LCP down.
Fri, 2011-04-08 12:22:24 - Connection terminated.
Fri, 2011-04-08 12:22:24 - Connect time 976.4 minutes.
Fri, 2011-04-08 12:22:24 - Sent 818101156 bytes, received 1250633158 bytes.
Fri, 2011-04-08 12:22:24 - Enter: tdb_store.
Fri, 2011-04-08 12:22:24 - tdb_store: calling tdb_update.
Fri, 2011-04-08 12:22:24 - Enter: tdb_update.
Fri, 2011-04-08 12:22:24 - Doing disconnect
Fri, 2011-04-08 12:22:24 - Exit. Fri, 2011-04-08 12:22:24 - Exit.
Hopefully Cisco / Linksys are paying close attention to these posts and are fixing the issue as we speak 
I seem to have resolved my disconnect issue. My router seems to have held an internet connection for 3 days. How I managed to resolve the issue is uncertain. Here is what I did:-
1. Disabled access restrictions. i.e. Access Restrictions > Internet Access Policy > Status = Disabled.
2. This didn't work so I disabled UPnP under Administration > Management > UPnP = disabled. ......I then re-enabled Access Restrictions
The router has been up for about 3 days now with no apparent issue. The reason I say I am uncertain of how I fixed it is because I re-enabled UPnP and the internet connection is still a non-issue. I would click save after enabling or disabling each feature.
What drove me to look at UPnP settings was that I noticed that when there was no internet connection there was always a DSL connection being reported by the router. eg..DSL status light was green and the router was reporting a line speed for up and down links. It was only the internet status light that would remain extinguished. e.g. The modem status under the stats menu in the router would not report any IP addresses or even a DNS. I use Windows 7 on my PC and I was curious if I could see my router on my network. I used Windows Explorer then clicked on Network which showed my router icon.....I then right clicked on my router icon and then clicked on 'Enable', which immediately restored my internet connection (I had a web browser open and I got it to go to my homepage). I don't seem to have any issues with disconnections after this. It was after this action that I re-enabled UPnP on my router....still no disconnections.
I may just be a fluke that my internet connection has stayed up for this long, so stay tuned.....
Still testing my router configs.....it did disconnect 2 days ago but I am not sure if it was due to the reported fault with my ISP around the same time. My router does appear to stay connected longer than 2-3 days. I wish I could say how long but I haven't retrieved the logs in time and the email alerts are pretty limited (I wish it could email upon a full log, Access restrictions website block. or other user defined events).
The connection is staying up longer with UPnP disabled. Still not 100% sure if this is because of UPnP and Windows / third party software during a shutdown process. I haven't experienced the router disconnecting while I was using it.
My observations so far:-
* With UPnP enabled in the router settings I am able to Connect / Disconnect the router internet connection from within Windows Explorer under 'Network' (a device icon is present with UPnP enabled).
* When the router disconnects, the ADSL connection is still up. When you check the 'Status' section in the router the 'Modem Router' section indicates that connection are down but the 'DSL Connection' indicates that the connection is up....in other words the router is connecting but is blocking/disconnecting the internet gateway. In this disconnect state the Connect / Disconnect buttons in the router settings don't appear to do anything (even if waiting for 5 - 10 minutes after clicking connect). A hard reset (power cycle/reset) is the only way I know of to get the router to connect to the internet. This is still looking like a firmware issue to me. The 'Keep Alive' setting only keeps the DSL line up....it currently does not appear to keep the internet side of things up. I would expect the 'keep alive' setting to keep the DSL and internet connections up at the same time (like I have experienced with other different brands of routers). I would also expect UPnP to have no bearing on keep alive settings. If Linksys / Cisco staff are reading this.... Linksys / Cisco should fix these problems in the next firmware release. It is a bit disappointing to see this type of problem with this brand.
* The router logs are cleared when the router is power cycled. Initial dates are reverted back to the year 2000 and time resets to 00:00. It is only after the router does an NTP lookup after about 5 - 15 minutes that the time and date corrects itself. It would be nice if the router could have manual NTP server settings added by the user.
My suggestions to try out (for Windows 7/Vista users):-
1. Enable UPnP in the router and make sure that the device is showing Disable under Windows Explorer when your right click on the router icon. If Disable is not present then click on Enable on the icon. Disable UPnP in the router after doing this and save settings.
2. Ensure under the 'Setup' tab in the router settings that 'Keep Alive' is selected and then saved.
3. Keep a copy of the router logs if you like.
4. Save router settings and hard reset (power reset) the router. The router should connect normally, assuming the correct ISP settings and correct username & password is entered and saved.
I am not saying that the above suggestions will resolve the disconnect issue as I am still testing router settings to see if it does. Regardless, I don't see why UPnP should cause such a problem.
Please note: Ensure Wireless MAC restrictions and Access Restrictions are not set to block your PC's IP and/or MAC addresses from your network....just in case people overlook these settings 
.
It would be interesting to know if there are any Linux / Macintosh users are experiencing the same problems with this router (please post your experiences if you do). If the above suggestions resolve connection issues then please post that too.
