M2-ATX power (sometimes) but no boot

I'm bench testing my setup:
Intel DG45FC with Celeron
1GB DDR2
VoomPC2
Seagate 250GB 2.5" SATA (disconnected at the moment)

When plugged into the AC-DC PSU directly, the system will boot and go into the BIOS configuration (jumper set for this). Everything checks out OK.

Now, when I have the PSU connected to the M2 ATX (12v constant, 12v switched, gnd), the system will appear to power up most of the time... the mobo light will go on, the fans will run, but it won't boot. Other times the fans will spin for 2 seconds and then cut off.

There's a strange ticking noise coming from the motherboard speaker when being powered by the M2. The only thing I can think of is a ground leak or something. Any recommendations or thoughts?

Got it taken care of already.
The voltage on the 12V rail was hovering around 11 - too low for the M2 to supply power. The ticking/clicking may have been a relay firing, or it was some power switching on and off, I'm not sure.

In case anyone else has the problem, the solution was to put a load on the 5V rail with a disc drive or any other peripheral. That, for whatever reason, boosted the voltage on the 12V rail.

The other solution is to get a better power supply that doesn't exhibit these quirks ;)