low FPS

hey guys i got some problem, its first time i have it, so wen i play every like 1 minut my FPS go down to 10 and i starting lagging, but it continuing for like 10 seconds, it never heppent before , my graphic card is fine(Nvidia GeForce 8500)
i dont know what to do please help

Close any programs (Browser, MSN, ICQ or whatever)