Troubleshooting an issue with my HD3870

[LTE]

So, my PSU recently died, so I grabbed a Corsair single-rail 650W psu for my rig:

Athlon X2 245
4gig ddr2 1006ram
AM2+ mobo
Win 7 Pro
HD3870X2

The old PSU died more or less from burnout--I think it straddled that edge of being enough power for the system.

About a week into use of the new psu (with no problems, running like a dream), the whole system just turns off mid-game. I hit the power button, and one of the case fans gives a bit of a wiggle, and nothing more. Pop open the case, hit power, and nothing inside stirs. I try it again half hour later, and the first attempt gives the fan wiggle, nothing more, and attempts immediately after don't even give the wiggle.

I pull everything out so that only the cpu is on the mobo, and only the mobo is connected to the psu. I hit power...and it fires up like normal. Connect the case fans...and it fires up like normal. Connect the HDD and ram...and it fires up like normal. Plug in all the peripherals, plug monitor into the onboard video (HD3200...quite the step down)...and it boots like a dream.

I plug in the GPU, making sure both 6 pin PCI power plugs are connected properly, and hit power...just the fan wiggle and nothing more.

I haven't tried using the 4 pin power connecters into a converter to 6-pin, just in case the particular 6 pin psu cables are bad. Short of that, I'm assuming I'm the proud owner of a dead gpu. Am I? And if so, any idea what went wrong?

 

[LTE]

So, I tried using the converters, since the cables for the 4-pin molex connectors are different than the cables that lead to the 6pin connector (referring to the actual physical cables that lead out of the power supply to the connector).

No dice. Like before, and watching the power supply specifically, when I hit the power on button, I get a quarter second of spin up, then nothing. Even unplugging all peripherals and case fans, such that only the gpu (well, and mobo, of course) is fed power. Nothing still.

Is there anything else to try to rule out the power supply? I'm pretty certain it's the gpu, though I don't know why it'd act this way. Unfortunately, I don't have a spare power supply or gpu to test on.