I'm running Windows XP by the way.
Just this one card in your PC?
What PSU? Please specify wattage, make, and model. Saying just something like "550 W" isn't enough.http://www.microcenter.com/single_product_results.phtml?product_id=0376717
(Not the place I bought it)
Has the card been overclocked?
Did you ever modify its BIOS (eg. the shader unlock trick)?
I had exactly these problems for weeks before I tried to unlock the shaders.
The card is fairly new and turned out to be a non-reference model. Didn't even have the dual BIOS switch.
The unlock didn't work, probably because some of the newer 6950s seem to be properly locked.
I restored a backup of the original BIOS after it became apparent that the unlock wouldn't work.
Have you tried running Furmark? Do so and report whether it crashes your GPU as well.
Was in a bit of a hurry, so I only ran it for 15 min. I guess I could do it again for a full hour if needed.
I changed the resolution to 1280x1024 and ran it. The GPU temp reached 95c after about 10 minutes. While mining, it never goes higher than 88.
The test should take at least an hour.
My computer crashes as soon as the miner gets to the hashing part. Shouldn't a GPU stress test cause the same problems just as fast?
Sounds like a driver problem to me. Downgrade!