It sounds kind of similar to what happens to mine occasionaly. If I turn the PSU off while doing some inside work etc and then start the comptuer cold, my system powers up for a few seconds, at pre-POST it 'turns off' again and then restarts and continues to boot normally. It only ever happens after the MB has been without power for any length of time. I too am overclocked and can't remember if this occured at stock speeds.
I also have it connected through a surge protector, but I don't switch that off.
As to what causes it, I don't know. I just put it down to being a feature, not a bug