My system has developed the same problem and I'm already on my second mobo. It ocurred after I changed some BIOS settings as well.
Core i7 920
OCZ Reaper HPC DDR3 1800
700 watt PSU
Have you tried setting all the BIOS settings back to default?
Also it might be worth clearing the CMOS battery.
Yes I have attempted to clear the CMOS with the jumper and from within the BIOS. Pulling the battery is a good idea, I will post my findings.
I am having the same issue with DX58SO mobo after doing a couple of bios changes, now it happens everytime even if I reset my bios back to defaults.
Pulling the CMOS battery fixed this for me. Give that a try.
Pulling out the cmos battery resolved the issue on my machine, plus all my settings were still set in the bios so I didn't have to change those back. Thanks for the advice.