Did you ever figure out the cause of your problem? I am going through a similar situation- my computer will black screen completely randomly it seems. I have a 7950 as well.
I did, for myself it was due to the power-saving features I had enabled. Apparently 7900 cards don't like turning off the screen too much lol. I had my computer all set to High-Performance power mode and everything, but I had the monitor set to shut off after about 20-30 minutes. When the monitor shut off, the computer would freeze up and black screen and be unable to turn the monitor back on for some reason.
All I did was turn off the feature that turned my monitor off after 30 minutes and set it to never turn off, and my computer hasn't done it since. Its been running perfectly for weeks now.