Set speed fan and check the temperatures.
If it's actually overheating just like this, I'd check all the fans, clean the fans and coolers and make sure everything is properly attached.

If it's not overheating, then my "expertise" arrives at some kind of dead end where I'd suspect a hardware failure of for example the graphics card. Which reminds me of Whacker's famous words that the 8800 series and some other NVidia cards may be made of bad material that can fail after some time (months, years) etc. Just a suspicion, I'd check the temperatures and interior of the case first.