I recently ordered two HIS IceQ X Turbo 6950 cards for use in crossfire mode. When they arrived I was currently using a 6870 Frozr II without any issues. I installed the two cards and booted up just fine. I'm using dual screen 30 inch 2560x1600 monitors. Everything seemed to be working fine until I loaded up any games such as bf3 or testing software like furmark. At that point the secondary screen would begin to flash black randomly every few seconds. In one case the primary display even began to flash to black in the middle of playing BF3.
At this point I took the cards out of crossfire mode and tested them individually (while both were installed) and had the same problem with mainly the secondary screen flashing but on occasion the primary would as well. It didn't matter which card I was using.
I then tested the 6950 cards individually removing the other card and even swapped PCI-e slots and power cables. All variations resulted in the same flashing and black screening. The temperature of the cards never pushed past 74C.
At that point I thought it might be a driver issue and because I'd been wanting to start clean for a while, I decided to completely format and reinstall windows starting fresh. After the fresh install of windows and after downloading the 11.11 catalyst drivers, the problem was still there. I tested again after the fresh install with crossfire, single cards, different slots, different power cords, etc. All resulted in the same flashing and black screening.
Just to test again I then took out the 6950 cards and put old 6870 back in again and everything worked fine.
I'm running an MSI p55-gd65 with an i5 750, 8gb ddr3 (4x2gb @ 1600), and a corsair TX750 PSU. As a last test I tried unplugging some hard drives and running with only two sticks of ram but that didn't have any effect. Do I have two faulty cards or is something else in my system causing this problem? I've seen a few others having somewhat similar experiences with black screens but its never the same cause with some returning cards and some buying new power supplies.