My girlfriend is having similar issues with her 8800 GTS. Her machine has displayed weird things when X came back as well. We thought it was the power supply, tried a new one and the issues came back, thought it was a heating issue, reapplied thermal paste on the chip set, and it came back. We tested the card in my machine and no errors came up. The only thing left we have to blame is the driver.
Look at your dmesg after you crash does something like this appear?
It seems to be an error a lot of people are getting and theres a hole thread about it on nvnews.net Some people saying downgrading to 100.14.09 fixes the problem but of course being that we're on Gentoo we updated X and we don't have time to do further testing till the weekend.
NVRM: Xid (0001:00): 13, 0003 00000000 00005097 00000d00 3f800000 00000004
NVRM: Xid (0001:00): 30, L1 -> L0