Had a similar issue here on campus, since we have three 360 consoles in the house. You can read up more about it, but basically you should install DD-WRT firmware on your current router, instead of whatever came on it. With this you have a bit more control over your network. Within that firmware you can enable uPnP, and set all your 360s to automatic network settings, this should fix the problem. It fixed ours for sure, I dealt with it last week. Strangely it was never a problem until modern warfare 2 for me either.
For what its worth we went from using the firmware that came on our Linksys router to DD-WRT, not sure what brand of router you use but the same should apply.