Your ram is 1.5v ram. Just saying. I would give up on doing prime. Download and make a memtest86 boot cd and run it. Im going with your ram is bad.... That or you may need to manually set your timings, some ram is picky about that. Either way you have a bad piece of hardware somewhere. If it passes memtest then its either cpu or gpu, gotta narrow it down....
I think it's board + GPU combination.
I've recently discovered than when C3/C6 states are Enabled, the GPU fan spins up and down quickly (just like when you bootup after a CMOS clear) between periods of 15' (eventually dies with driver error or freeze) while running Windows.
Also, Linux eventually crashes (e.g: video frozen while other activity ok).
This does not occur without the C3/C6 states.
From this description, what about some form of PCIe power problem?
My brother has a good ol' HD4850. Maybe i'll take that and try.