Dirt 3 HD6950 2gb Fault or Bad PSU? PC SHUTS DOWN!Help?

Hey all, about 1 week ago I purchased a Dirt 3 edition HD6950 2gb VGA from Newegg and installed it over a fresh copy of windows 7 64bit, into my pc build. Upgraded from a XFX5770, big upgrade, but this past week Ive been having shut down problems. I recently did a motherboard bios update to support my new AMD Phenom II x4 960T, untill now, the CPU temps remain fine, no temp problems , Im running a watercooled Antec H2o 620,The GpU temps are also under 65 full load, PC boots fine, I can run it hours without any demanding apps, or 3d apps, but for some reason, when i stress test my GPU under Furmark, ( i know its very demanding and gives unrealistic loads) it will run fine for about 15 to 25 mins, then if I alt-tab to acces another app such as MSI Afterburner my PC just cuts off, power out, then as I click my power button, nothing happens, maybe like a 2 -4 minute wait to turn on unless i disconnect the PSU cable for about 10 seconds, then it'll boot up again, This also happens when I play a game for ex. BF3, or Saints Row3,etc. it will run fine for a while then just randomly shuts off. Ive tried the same thing with my 5770 under max loads and high OC, but no shutdowns with this card, then again the 5770 uses much less power than a 6950. Now im stuck thinking if this is either a PSU fault or a GPU fault, but my guess is PSU , its a lilttle over a year old and if it was a GPU fault, the monitor whould just shut itself off not the whole system right? Its driving me crazy   , and i Have 15 days left under neweggs RMA policy for my VGA to find out whats wrong here. Any help is greatly appreciated....Here are my specs

Gigabyte MA785GM-US2H
AMD Phenom II x4 960T Black Edition
Antec Basiq 500watts 80plus 18 amps on both 12v rails
Antec H2o 620 with 2 120mm fans on Radiator
1 IDE 150GB HDD
1 SATA 500gb HDD
1 CD/DVDRW
2 color cathodes running of molex connectors
4 120mm led case fans total of 6 120mm fans
1 Antec vga slot cooler
2 800mhz DDR2 AMP RAM=4GB
2 800mhz ADATA RAM=2GB
HD 6950 2gb Dirt 3 Edition, shaders unlocked