Anyone else have this xbox live problem since buying a router and what is it all about?

before when i connected just an ethernet cable from my modem to my xbox the games worked perfectly on live hardly ever a thing going wrong but ever since i buy a router to connect my pc and xbox at the same time every time i playa game it always f*cks up after about half a minute every single time. is this common and how can i fix it??? my router is a belkin