GA-990FXA-U3 with Sapphire 7970 = no graphics

[cordless]

I have a GA-990FXA-U3 Rev.4.0 that I am trying to setup.
BIOS: F2
PSU: Seasonic X-1250
CPU: AMD Phenom II X6
RAM: Corsair DDR3 1333

I have two Sapphire Radeon HD 7970 Vapor-X 3GB Ghz edition graphics cards that I am trying to get working, I am just getting a black screen, I have fans and lights but that's it. I have been trying with just one card in the first X16 PCIe slot. Two 8pin PCIe power adapters are plugged in to the card as required. If I take out the 7970 and put in a PowerColor Radeon HD 7750 the screen displays and I can get in to the BIOS settings. I've tried resetting the CMOS as well. I'm assuming the graphic card is not the issue as they are both brand new and show the same symptoms.

What am I missing, is there a BIOS setting I need to change for those cards to work while the 7750 works by default?

Thanks in advance for any help!

[cordless]

So for anyone else who runs into this oddball problem... Some of the PSU's main motherboard connectors weren't working properly. No idea how that causes the problems I was seeing, but that's what fixed it.... Maybe the card couldn't pull the needed wattage through the PCIe connector as it wasn't fully powered?