I'm also having the same problem with my P5E WS Pro and GX2 setup. I thought it was a PSU issue since i used the 6 to 8 pin adapter but today I changed the PSU to a Corsair TX750W and the problem remains.

I'm really getting worried about this, i already PMed SIOUX since he had the same board but he told me his was working fine so it seems that it only happens to some boards