I have the fix - I believe..!!!!
Thsi exact thing happened to me. I went through all the checks, took out the MB to check for shorts, removed all connected peripherals. reseated the CPU and RAM.
I contacted overclockers who told me to RMA the MB. It was checked and found to be fine.
Rebuilt the M/C - SAME PROBLEM.......
THEN.....!!!!! I took a big breath, read the manaul again (RTFM) and realised the OBVIOUS....
I had only connected the 24 pin power connector and NOT the 8 pin as well.
I had exactly the same symptoms as you did previously - THIS WAS THE SOLUTION...!!! THERE ARE TWO CONNECTORS TO BE FITTED....
I hope my stupidity will mirror yours and that this fixes the problem for you.

This is the fourth PC I have bulit - I've done this stuff before and should know better. I'm still sheepish at the thought!!!
Regards
Paul