I posted this same thing to mrmeanr, it might help you as well:
I was having tremendous difficulty getting my HD 7870XTs to crossfire properly, I was getting all kinds of system problems and destabilization. I took my machine into NCIX in Mississauga and the tech there, Dave, he was about to RMA a second set of cards when he had a hunch. He changed the PCI-Express power setting to "Maximum Power Savings" and, for some reason, it immediately fixed everything. No more freezing, no more crashing, everything stable. Just from that ONE LITTLE HIDDEN SETTING. Try it, it might alleviate your problem as well.
Control Panel -> Power Options -> Change Plan Settings -> Change Advanced Power Settings -> PCI Express
All hail David the mighty for having solved this problem! The man is a genius because I never would have thought to do that.
Intel is SATAN and nVidia sits at Intel's left hand.