My modem's ready light keeps turning off and I keep getting dissconnected from the internet, how do i fix this?

When I am playing Modern Warfare 2 on my xbox 360 my modem's ready light will turn off and my xbox(which is connected to my router) and my computer(which is connected WIRELESS) will both be disconnected from the internet. This seems to happen quite often and it's just plain annoying. Can someone please tell me if its the modem and that I need to get a new one or if it could be the router or xbox 360s problem. By the way I have a linksys router and my modem says its trademarked under Ambit.