P5K Bios Graphics Scramble/Dots/Colours

My custom pc has been running fine for nearly a year now but in the last few weeks ive had major problems. It started when i was gaming, every now and again my game would just lock up and i would see colours scramble and flicker. It got progressivly worse untill the desctop would be distorted, and then soon after the bios start screen would scramble aswel. I was using a 8800gt, tried all drivers etc... so i bought a temporary 6800gt to see if it was the gfx card. Im now having the same kind of problem with this card aswell, which suggest it isnt the card.

Things ive tried:

Changeing graphics card from 8800gt to 6800gt
Trying a varity of different drivers
Fans/Temps
Both PCI-E slots in the mobo
Using a different psu to run the gfx card
setting bios settings to default
Reinstalling mobo drivers

The only other thing i can think of is flashing the bios but ive never done it before.

Rail voltages are:

Vcore voltage 1.176v
3.3v Voltage 3.264v
5v Voltage 5.136v
12v Voltage 11.928v

All my videos are on youtbe below

Boot (8800gt)

http://www.youtube.com/watch?v=CaUZYp3r2Kc

Safe mode (8800gt)

http://www.youtube.com/watch?v=1_B3ztSclVw

Desktop (8800gt)

http://www.youtube.com/watch?v=XN3JEKtlBv4

Boot and crash (6800gt)

http://www.youtube.com/watch?v=_StYFSofdQM

Please help, its driving me insane

Thanks