It flutters

I have a desktop that is a couple years old and worked fine with my old modern And a seperate wireless router. Then i upgraded my comcast to a router that had wireless installed so no need to use my separate router. As soon as that happened my computer stopped consistantly conecting to the internet. The connection flutters on and off constantly. Sometimes it will stay on for a couple of minutes then disconnect, other times, most of the time it connects and drops service so fast you can't get it to go to a new page. What is wrong? why the sudden change? And no i can't call them back out i waited to long.