Strange power issue

[listmash]

I am having a very weird problem.
After shutting down my computer normally, it refuses to start up again. Case fans, cpu fan all run, but nothing else. If I turn the psu switch off for 10-15 minutes, then on again, things seem to start up. Same thing happens if it goes into sleep mode, but I disabled this to get around it (I know, not a good solution).
I though it was a psu issue, but I tested it with a good multimeter, and all voltages are within ATX spec. Now I'm thinking it may be a mobo issue, but don't know exactly what to check for. Cleared the cmos, but no change.
Any suggestions out there?

System is:
Win7 64 bit
Gigabyte mobo GA-Z77-D3H
Intel i5-3570
Corsair TX650
3 hdds, 1 ssd

Any help would be appreciated.

M

 

[knightdog56]

Check ALL! connections from power supply to motherboard and other components make sure they are secured and locked. Also reseat memory modules, take precautions for ESD. Make sure power supply switch is in the off position.

 

[listmash]

All connections check out. 2x8Gb memory modules are both seated properly and in the correct slots.
This problem might make me insane!

 

[knightdog56]

Could try to clear CMOS, and then go to BIOS and Load optimizd defaults. Also read this article it might help you find something you may not thought of. http://www.hardwaresecrets.com/article/What-to-Do-If-Your-Computer-Doesnt-Turn-On/547