In the last few months of my 7950 crossfire setup, I had quite a few of these crashes under various scenarios. When I got GTX 970, and moved one of the 7950s to the HTPC, I found that the card was defective. It could only post video in BIOS, safe-mode or Windows logo screen - basically anywhere, with lower resolution. In Windows the card would just show black screen.
Now, it was the secondary card on the crossfire setup and the PC worked as expected in games with higher frame rates, except for strange crashes here and there on desktop. I ran the card as single GPU when I purchased it, and it was fine back then.
Sapphire and AlthonMicro handled the RMA quickly enough and I got a replacement 7950, which works fine on the HTPC.
I wonder if all (or most of) the crossfire issues I had on Windows desktop were simply because of this broken crossfired card. Someday, I'll put the two 7950s together again and verify.