I had similar problems, but lonly happened when i was playing CS :S or Joint ops (esp joint ops). Clocking down to 375 solved my problems, for the most part but from reading various forums it *might* be a hardware conflict with usually a audigy 2 , and in my case probably my wireless controller or my onboard LAN chip. Also I would caution to think that all GT chips can do Uttra speeds. The Ultra speeds are attainable most of the time, but not all of the time. I suggest you do the following:
1) clock (core/mem) 375/1000 run it for a few days or stress it and see if its stable
2) clock 382/1000 run it and stress it (esp if it crashes on certian games) see if its stable
from there on out go 5mhz up to 400 if you get instability then take 2mhz off till it is stable
for memory
(stable core OC/memory) -/1050 test for stablitiy
-/1075 test for stablitiy
-/1082 test for stability
5 mhz up to 1100 then stop when you start getting lockups or artifacts ( if i remember correctsly memory tends to less forgiving than core, so you might get lockups as opposed to artifacting)
Let me also add that turning off fast writes did not help me (i used riva tuner) and also let me add that I have a GAinward 6800GT GS and the card is still flakey, short of a new os install with their recommended drivers I cannot say for sure whether its the card or the OS + drivers not gettign along...