I noticed when I unlocked my cores on my x2 550 and I power off, I must flip the psu switch to POST again, otherwise everything will spin and HDD turn on but no POST. I have been doing this for weeks too, and didn't even notice! It is always off when I do not use it, and I always flip the switch because then my sisters can't use it. =) It was completely stable. But I didn't up the voltage to see if that would work, as was suggested to me.
So I have been keeping it in Dual mode. It POST's now, and I can overclock without problems. My mobo is rated for 140W cpu's.
I wanted to ask before I did anything, see if this has happened to anyone else.
More aboutpower problems quad
Your PSU probably has a problem with the standby power circuits in it.
JSC is definitely on the right track. This is the problem 90% of the time. Simple check would be to swap out the PSU with a working one to test the issue. If you still have problems it points to the mobo.
I thought for sure it wasn't the psu because it will work fine when I am using 550 instead of B50. Well, all my spare psu's are ones I have obtained from broken HP's and are 250-300 range and filled with dust.