what CPU do you have? Also, you should know, the gtx660 cards throttle back if they get too hot, check the temps and make sure you have a couple exhaust fans to suck the heat out. And what PSU do you have? maybe the card isnt getting enough power. I actually have a similar problem in crysis 2, its happened a couple times, its like the card goes back to idle clocks. It only seems to happen in multiplayer if my ping is too high. I just restarted the game to fix it. Hasnt happened for a while, make sure your drivers are up to date too.