Probems like these are usually down to 1 of the below.
1. Overheat. which i agree with others is the most likley problem.
2. drivers corrupted
3. card not seated correctly
4. PSU insufficiant wattage or failing under load
easiest thing to do is check the GPU temp. get ntune or ATI Tools ( yes it does work with nvidia)
get latest drivers, even try rolling back to older ones
reseat card, make sure to push in all the way home
psu is the harder to diagnose, try running some stress/stability tests.
other than those, cannt think of anything else
Oh, and finally
Go back to XP, god i hate vista P
Good luck, and hope it helps