New thermal grease always helps. But the card you own should be new enough to where the thermal paste hasn't dried up completely, some of the more "generic" vendors use the worst thermal paste you could immagine.
Run Rivatuner's Hardware Monitor in the background, keeping track of temps at all times. That way, when you've been playing a game for about 10 minutes, you can Alt-Tab and see what the temperatures are like.
8800GTs should run as hot as 100c, without crashing (hopefully). But, it would be preferable to keep temps around 70-75, if possible with the stock cooler. Normally you have to manually turn the fan speed up in the control panel. The fan speed doesn't automatically go up on a lot of the newer cards, they are scared to let you know how loud the fan actually is I guess..... So, you end up running your card at 90c, and your fan speed is still 40%, which is bad. Turn the fan speed up to 75% once (it should be very audible at this point), and keep track of the temperatures.
Stress the card for a little while, and tell us the temps. Maybe try the new BETA driver that nvidia has out right now (it's on TPU's front page).
When you install new drivers, you uninstall the old drivers first, right?? AND, you uninstall any previous drivers from any other video cards too.
Oh, yeah, and also, I had a similar problem with my x800, on an older motherboard. I updated my motherboards BIOS, and everything was fine... try maybe updating your motherboards BIOS too. After all the 8800GT was designed for PCI-E 2.0, and that wasn't even out when you purchased your motherboard, so you may very well need a BIOS update (not that your mobo will support 2.0 with the update, but it might help.)