Radeon HD 7850 Power Down on Startup

I just bought a new GPU (Sapphire 2GB Radeon HD 7850 OC) and installed it in my PC. When I powered the machine up, it ran for 5 seconds, powered down, then attempted to power up again, only to quickly power down. It kept repeating this cycle until I flipped the switch on the PSU (Raidmax RX-500AF).

GPU: http://www.newegg.com/Product/Product.aspx?Item=N82E168...

PSU: http://www.newegg.com/Product/Product.aspx?Item=N82E168...

When I unplugged the PCI-E 6 pin power connector the PC started up with no problem.

It would seem that I've got plenty of total watts (500) and amps (each 12v indicates 20amps). The PSU indicates that the combined watts from 12v is 384watts which seems like enough for this GPU card.

Any suggestions?