PSU. Yes i know it says 800W, but the card needs 1x6pin and 1x8pin rail. I know you can use two 6-pin - 8-pin adapter but good luck with that, no guarantees it will be stable.
"Yeah i'm using the 2x6pin-8pin and a 2x4pin-6pin connector supplied by BFG", this is what you said. Get new PSU and your problem is going to be fixed. I suggest you look at Rosewill 950W PSU, it's only $150 but it works damn great. It has 2x6pin and 2x8pin PCIe cables, 1x8pin CPU, and ATX24 pin, and plenty of SATA and 4-pin molex cables.
It's $169 now, my bad.