The 6800 will produce much less heat.
Try putting your 8800GTX in the oven. That seemed to fix a couple of them, believe it or not.http://www.ninjaforum.com/viewtopic.php?f=32&t=1316
If your 8800GTX died, there might not have been enough thermal paste on, or something. With a fan blowing directly on it all hot air would be pushed away, so there's no explanation other than faulty hardware.