Is This A Power Supply Problem?

I built a new PC recently and it has never worked properly. It would freeze up regularly. I've RMA'd the motherboard twice and the CPU once. I finally gave up and took it to a local shop for a diagnosis. They said it was the motherboard as they swapped out everything else. So I switched motherboard brands to see if that helped. Now it won't even power up. So I took it back into the shop and now supposedly the power supply is bad or incompatible as it powers up if they use a different power supply. It also powers up if they use a video card that doesn't need extra power.

Does this make any sense? Here are the specs:

Antec 900 case
GIGABYTE GA-P55-UD3R LGA 1156 Intel P55 ATX
MSI R5850-PM2D1G Radeon HD 5850
OCZ Gold 6GB (3 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600 Low Voltage Desktop Memory Model OCZ3G1600LV6GK
ZALMAN ZM750-HP 750W
Intel Core i5-750 Lynnfield 2.66GHz
Windows 7 x64


I swapped out the Gigabyte motherboard for this one:

ASUS P7P55D EVO LGA 1156 Intel P55 ATX Intel Motherboard


Does it make sense that the power supply would have problems with a different motherboard?

Any thoughts or suggestions would be appreciated.

Sounds like the diagnostics have been done already. PSU fault seems correct to me.
If they got it running fine with a different PSU, and that was the only part not swapped out last time, then I'd say that's clearly the issue.

Some computer parts are more tolerant than others to voltages etc.
You could RMA the Zalman or try a different brand.