I had six cards working on the mining rig before. It was working fine. I tried 13.1, 13.3, and 13.4. All crashed.
I suspected it might be the motherboard as well but it can start the other GPUs just fine. So that's when I suspected it was the two GPUs until I put them in my office computer.
If you just put the two GPU (which is in office computer) into the mining rig, does it work? If it works, then add more GPU on the mining rig. I suspect something might be wrong with the mother board.
Can you try Catalyst 13.4?
Does it happen more on LTC?