Unknown gfx card problem

Recently I've had the most random problem with my system. It started when I was playing crysis high settings max reso on my monitor (1280x1024) steady for 1 hour or more, and the screen went black. The monitor says no signal after 3 seconds, then the system switches off. 

My computer the gets stuck in a restart loop, where it keeps on restarting either at the post screen, at the windows logo loading screen or as soon as the ari drivers on the desktop load.
 I've tried Booting into safe mode which will still randomly restart after a few minutes. I've triedreinstalling the drivers including using driver sweeper with no luck. I kept it on long enough to flash the bios, which worked for 2 days of normal use then the problem persisted. 

This all happened a week after swoping out my 4850 for an asus 4890 ( one with twin fans ) a week ago. I know a lot about computers but even I'm stuck! Please help.
 
My build:
asus p5q se2 mobo
antec 650 watts psi
asus 4890 gfx card
intel core 2 duo 3 ghz processor
corsair 2x 2 gb ram
windows xp sp3

the system itself apart from the gfx card is 10 months old