Hello,
i'm having a real weird issues with my rig. Random resets and a reproducable reset with Furmark and certain settings

The PC resets within 10 seconds sometimes without even showing any picture.
Hardware:
MB: Gigabyte GA-EX58-UD5
CPU: Intel Core i7 965XE
RAM: 6x2GB DDR3-1600 Corsair Dominator
GPU: Gigabyte HD4870X2
PSU: beQuiet 1200W
Raid: ARECA ARC-1220
HDD: 2xOCZ Core Series V2 120GB RAID-0
6xWD 1TB RAID-10
Software:
Vista X64 SP2 Ultimate
Catalyst 8.12 - 9.6 all the same.
GPU-Temps never hit 100°C however while resetting the R819-LED on the GPU(indicates overheating) lights up on the old as well as the new card.
Ram was tested with Memtest for over 8h without errors as well with Prime95 for 15h.
I already RMAed the GPU, PSU and MoBo... and now I start running out of clues.
Any advice appreciated.
Regards,
morphly