Access the setup page of the router, go to the "MAC address clone". Enable the MAC Clone and click on Clone Your PC's MAC, then save the settings. Do power cycle on the router and the modem. Then check if you can access the internet.
My wrt300n is working ok with comcast. I think the most obvious thing is that your linksys should be configured to use dhcp for the wan. Otherwise, when your comcast dynamic IP changes, things won't work anymore.