Also, another sure sign is, if it knows it's dying, it might well be writing out a will. I suggest carefully check around to see if there's a copy of it on your PC's harddrive!
Serious reply, try GPU benching software like 3d mark. Also, are you sure that it doesn't have a cake load of dust in there? Check to make sure that it's not overheating - or better still just give it a clean anyways.
You could also try taking the side off and aiming a desk fan in there (I've done it before just to see if it happens).
Are you sure that oblivion isn't corrupt, if you've got a bunch of mods they could be causing issues.
*edit
Hmm, looking at your reply this post is kind useless.
You could try underclocking the GPU core and see if that fixes it. Sometimes, if its on its way out, down clocking is a temp fix and can kind of give that it's the issue.