Game Problem !!

Hey guys, I have a problem with the Crossfire game !
First, I installed the Window 7 and installed the Crossfire, and I played Crossfire is normal, but for some reason now, After I've played Crossfire game for couple rounds , my screen began to lag ( not bcause of internet ). My internet is good. but the screen is lagged. Any Help ?
p/s: I have tried clean install Vista + win 7, in both window the screen is stilled lagged for after couple rounds in the game. The screen is not lagged for first couple rounds, but after that, its lagged . Again not lag of internet, but lag of the screen ! I've update my graphic card up to date !