Xbox 360 Black Ops lag issue?

I reset the system, router, and modem and my games go good for about 10-15 mins than every game after is trash. It seems like I am lagging. Please don't answer and tell me to get better I am lvl 6 prestige. I have played alot and the first couple of rounds I get 20+ kills with about 10 deaths and than after that it almost reverses to 10 kills and 20 deaths. This is a question more for techies.