Imac flicker/blink then crash

Imac 24" 8,1
nvida 8800 gs
3.33GHz Intel Core 2 Duo
4gig ram


Problem: During almost anytime while its on the screen will do a flash (whole screen like a flicker) after that the computer is non responsive, can move mouse cant click/drag on anything force quits wont work. Managed to run Spore with horrible graphics for about 1/2 hr no problem thats the only program i know of that it wont just crash on me. mail/firefox/safari/small games are out of the question and has crashed with nothing at all open.

Tried:
Did Extended hardware test : no problem found
Reset PRAM on start up : no change
Reset SMC (also got SMC controler and set the fan on high no change)
Disk utility verified permissions: nothing
Ran Clamxav (virus prot) : nothing
Ran and compared tempetures: seem normal
Tried to wait out the crash 15-20 mins: never came back
Cursed and threatened computer for an hour: no response

please help point me in the right direction im very confused, seems to have started about thurs/fri whenever they updated safari (not sure what else was updated or if that would effect it)
Applecare ran out :*( already paid $2600 not looking forward to paying more

 

:*(

really no one knows anything about it? would punching the computer help?

 

still nothin?