I can't for the life of me figure out why this isn't working.

Same model cards, both HD5850's (though diff manufacturer, Sapphire and ASUS)
Work perfectly fine independently
750W PSU, cards are powered with seperate PCI power cords plugged into the PSU.
Crossfire enabled in CCC

When any game is launched fullscreen, atikmdag crashes.

I'm really lost here. Google only returns clean Windows install or RMA as possible options (besides driver reinstall, temperatures and PSU. Already tried driver, temps are no prob, and I believe 750W should be enough), and the problem is occuring on a fresh install of W8 (accidentally pooped out W7). Card is long over guarantee so RMA isn't a viable option.

Anyone know anything that I can try?