This is really odd! I have a virgin media cable connection - the local network remains fine, but the cable modem loses sync and requires a reset to reconnect. It happens anything between 10-30 mins as of yesterday.
I've kept an eye on the modem, and it stays sync'd if the xbox is off, though other devices are connected and being used.
Devices (irrelevant no doubt):
Xbox 360 slim 250GB - ethernet
ipad/iphone/vaio - wifi
Any ideas at all? :( It's causing me big problems playing COD! (lol!)