Sometimes it's little things you can't see. It took me a while to find why my pc was shutting down when my gpu's were running benches while Xfired.
Just came down to a couple of wires that had been worked so many times they were not making good enough contact with the pins...it took me days of testing and looking + disassembling the gpu cables to find the problem. So now I'm in the middle of making new gpu cables.

Hopefully if you keep at it you will find the problem, sometimes troubleshooting takes a long time but I usually figure it out in the end.