Gigabyte HD 3870 Power cable issue and Asus P5K keeps shorting

Hey guys,

I recently bought a Gigabyte HD 3870, Asus P5K and Q6600 quad core to sit on it.
At first I was repeatedly getting a bit flashing red warning message "You have not correctly connected the power cable to your graphics card" type thing using both a 1000W Thermaltake PSU and a 780W Ocz. I've tried every pin connector available, 8 to 6, 6 to 6, dual rail 4 to 4 x2 (using the provided pci-e cable power converter thing for older PSUs) to no avail.

If that wasn't the worse of it, my mobo now refuses to do anything, it powers up for about half a second then powers down, if i hold down the power switch it does this in a rapid cycle so I end up with a kind of funky strobe effect with the CPU fan LED :D
I tried taking the whole rig out of my case, unplugging everything other than the CPU and it's heatsink, and putting it on a foam block with the same problem. I shorted the mobo's power connector pins and got the same problem so I can probably rule out loose wiring of my power button, I even tested the PSUs (both of them) out on another rig and they power up fine.

Any ideas? I'm a total loss