The only thing I can suggest is to your video card bios, drivers, and make sure that you have ample case flow and no dust build up. Then install riva tuner to run your fan slightly higher and possibly bring some temps down if they are high. Also, a clean of the OS may help...such as running spyware, ad-ware cleaners, virus scan, and check for all windows and game updates then give it a good defragment after everything is done and see how it goes. I have never seen the problem, but if the above does not work then it would appear that it is most likely a hardware problem imo. If your card is overclocked try bringing it down alittle. Hope this helps!
Are you running the card o/ced? what's the temp on the GPU while playing? I don't wanna give you bad news, but I hope it's not an MSI card you got there. My brother owns an Internet caffe and he has 80 PC running MSI 7900GTX 24/7. Half of them crapped out after one year of operation. It seems that this particular model had a tendency to go bad after a while. Don't know if that's the case with other 7900GTX from other manufacturers. Better be (emotionally at least) prepared that soon, your card is going to meet its maker. I'm saying this cause when one card was about to give, they would show various artifacts while playing (remember, these cards where on 24/7 so it might be a few months before yours craps out, if it does crap out)
Sorry to hear it man i went through 5 of these things when they first came out in 3 months for my first Sli rig. Mine were XFX had 665Mhz core and 1.63Ghz mem i went though lots of forums when i got mine. I ran 3d Mark over them when i first got them during the canyon flight test it would produce random artifacts like bright blue and red stars coming of the pilots shoulders sometimes the side of the canyon would glitch a bit and then they'd fail the test or the test result would be way below normal if not it'd crash or BSOD. Finally got 2 running fine for the last 12 months sold one last week. IMO these guys are right but i'd like you to try running 3dmark 06 first. if it gives the same results as what i just said post some pics and get'm replaced under warranty if they still have any left. Good Luck.