PNY 8800 GT overclock: Unstable? Need help

Hey all,
So I got an 8800 GT 512MB recently, cost me $40 in Best Buy money after I sold my old, and now here I am. I tried overclocking my card but I am getting some odd results. I overclocked the card to 675/2000 and it worked fine. Next I tried to overclock it to 700/975. Here is where it gets weird. The card will be fine for about 10-15 min in CoH or Crysis, havent's tried any other games yet, and then the screen will lock up. Now I adjusted the fan speeds from stock to 90% and when I look at the temps I am not even above 60 c, so as far as I can tell it's not overheating. What I don't get is if that clock was too much for it wouldn't it be locking up sooner than this, not going 10 minutes and then locking up? When the game starts and is working at that clock it runs fine. I am using rivatuner to overclock. Any help would be great.

Thanks

Thing is I've seen people push the clocks I'm trying on stock cooling and higher. This makes me believe I had the misfortune of getting a lower quality chip, which sucks. I just want to see if there is anything else I can try.

I just tried Crysis and it turned into a slide show at one point. Is this normal? I even tried reverting the clocks to normal.

yeah, same card, same problem. best ive gotten stable was 675/1675/975

never been a heat issue, with stock fan i never got to 80, now i have a zerotherm c92 and i dont get over 70 loaded, just the hardware i suppose.

using rivatuner, may try ntune. But im not to hopeful..

considering i upgraded from a 7900gt 256, Im Happy