Crossfire freezes computer

[CapitalHitman38]

i recently got 2 R9 270x graphics cards that i can crossfire. but when i make any games fullscreen (to activate the crossfire technology) my entire computer freezes and the only way to get it to work again is to shut off the power supply/unplug it and plug it back in.  I have tested each card on their own and reinstalled the drivers. im running a brand new install of windows 8.1. I can open the game and then when im in, fullscreen it. but it eventually crashes again.

 

   

 

 FX-8320 @ 3.5Ghz

MSI R9 270X Gaming 2G x2

ASRock Extreme3 r2.0

6GB DDR3 

120GB PNY Optima SSD

Seagate Barracuda 1TB 7200RPM HDD

Corsair TX850M