Sounds like your having the same problem as me. Its annoyed the crap out of me. I just bought this graphics card and i wanna throw it. I'm thinking about taking it back, but they 'test' it and if its not defective they charge you 25$.... so i want to be 100% sure thats its defective before i take it back.
Ive ran the tests and its not a heat issue, everything is stock...
I'll trying putting in more volts into the PCI-E thing, you were spot on with the NB volts.
I hope that this is the case and everything is just being under volted..