It's taken me a while to isolate the source of the trouble, but it seems to be my switch.
When my xbox360 is connected into the EZXS16W switch it will always disconnect me from xbox live after a few minutes in the middle of a game. I then have to login again. But if I plug in directly to the BEFSR41 router, it works perfectly. No disconnects. Other devices (roku soundbridge, printer, computer etc) plugged into the switch don't seem to have any trouble. Just the xbox.
I have been running the latest firmware on my router.
Thanks. Actually, it appears that I am having the trouble when plugged into the router BEFSR41 v1.0 as well. The time period I tested earlier wasn't sufficient. This makes more sense as I couldn't imagine how the switch could be the problem. But I definitely have no problem at all when plugged directly into the cable modem bypassing the router.
I am guessing that the MTU proposed solution still applies. I will give it a shot.