Question : Problem: PSU fan behaving erratically, unprompted shutdowns

Here's the skinny.  I troubleshoot/repair PCs in a limited capacity, but I am new at it and this one is my PC.  I'll give you the upgrade background first, then the anecdote, then my troubleshooting steps.

BACKGROUND:
PC was originally an HP a6212n, Vista pre-loaded.  I managed to get XP on there and all drivers/sound functioning properly.  I upgraded RAM, installed a new Cooler Master PSU, and an NVIDIA 8800 GTS.  All of this went smoothly, and my problems are certainly not from a recent upgrade as all this is at least a few months old.  Just wanted to give the info in case someone asked later.

ANECDOTE/TROUBLESHOOTING
PC was in use earlier in the evening, I hibernated it; went back to use it later to find it off.  Tried to turn it on, no avail.  Swapped out power outlet/power cord, eventually it powered back on but not due to either of those.  Stayed on a few, used the restroom and found it off again.  Wouldn't turn back on again.

Went into a bit more detail--unplugged PSU's mobo connector, shorted out the green with a ground to get it to run.  Was looking for fan spin-up to isolate mobo/other hardware vs. PSU itself.  PSU fan spun.

Got PC to turn on again in normal configuration, noticed PSU fan wouldn't spin.


So at any rate, looks like for whatever reason the PSU fan will not spin and seems to be overheating when its plugged into the motherboard.  When its shorted out, it runs and the fan spins fine.  

I can't imagine what the issue could be; I guess I don't know enough about how this works to pinpoint why the motherboard being present could keep the PSU fan from spinning.  Any ideas?

Thanks much.
--Ben

Answer : Problem: PSU fan behaving erratically, unprompted shutdowns

It could be that the PSU works fine under no load but once the motherboard and other components are introduced the PSU can't perform. The fans on a PSU are usually 12v as are the main components on a motherboard.
I would replace that PSU as you don't want to end up frying a components due to a surge from that PSU