PSU glitch?

After collecting all the parts over the last 2 months I finally sat down to do an extenal build last night before putting it together in the case.

I was unable to get a BIOS screen (making me think GPU problem). I don't have a speaker to plug into the mobo header so never heard a POST beep if there was a signal.

I unplugged the GPU PCI-e and tested the voltage to see that cable was only carrying 0.5V (turning it into a PSU problem affecting the GPU). I reseated everything on the board, and got the same results 3 times.

MOBO: Gigabyte GA-990FX-UD3
CPU: AMD FX-4170
RAM: GSkill Ripjaw X 8Gb (2x4Gb) 1600
GPU: MSI R7770GHZ
PSU: Seasonic X series X650

I did manage to have time to test the PSU a little before work today, and it was putting out 12V on the PCIe cable. But havent had time to put the GPU back in to test under load.

Any experience wiht this? Was this a glitch? Odd grounding problem? Could it have caused any other problems? I'm not familiar with the inner workings of the PSUs.

First time builder, btw.