Graphics card problem

My system specs:
CPU: Intel C2D E6750
Mobo: Gigabyte P35-DS3
A-DATA Extreme 800+ 1 gig sticks, 2 of them
650W PSU
Soundcard SB X-Fi
Gfx: Nvidia 8800 GTS 320meg (162.18)

The graphics card fan runs at max constantly, the temperature is constant between 80 and 85 degrees celsius. The problem is that when the load gets a bit higher the screen basically loses signal, goes black and says signal lost. Rest keeps running, i can still hear the sound, it keeps running, and then 5 minutes later it all crashes. When i force-restart it there's is no beeping from the motherboard to indicate a problem, no errors when windows restarts. Loads everything fine and everything is good, until the same thing happens.

Any ideas?