Dunno I'm on the GA 790X-UD3 board and I fear it has some issues with 7970 cards in crossfire... single card it all clocks up nicely. In crossfire it was lockup galore, loosing display,... rig was very unstable...
We used Trixx and even registry setting to disable ULPS... reinstalled OS, tried other drivers flashed 3 biosses for the board... no go...
Pretty clueless besides to point the finger at the Giga board...