Same issue here. Very similar circumstances. I still have the same router, but I disabled it's wireless interface and added a wireless access point to my network. Mostly for better signal strength, and upgrading to wireless N.
I can be watching Netflix and it will pop up and say I've been signed out of Xbox Live. Neflix doesn't even stutter, so I know it's not actually dropping wireless signal or getting bumped off the internet. I can continue watching my program with no problem, only needing to sign back into Xbox Live when I want to select a new program.
No errors are displayed, just "you have disconnected from Xbox Live". Timeframe is random, sometimes every few minutes, sometimes an hour.
*edit* Crud, sorry, this was supposed to be a response to this thread. http://forums.xbox.com/xbox_forums/xbox_support/f/9/t/81386.aspx