Wow, glad to hear it's not just me having trouble with Clear Sky and 4850s.
I cannot run crossfire for CS using Win XP and any of the recent Catalysts. I get great FPS for awhile then will slow down to a slideshow. rinse and repeat. I have tried -noprefetch, the affinity mod, turning down suns rays, forcing vertical sync, and reinstalling of everything, even the OS and nothing helps. Add in that even when I run a single 4850, I get your symptoms of Black Screen followed by the VPU recovery 'Send Error Report' popup. I get VPU recovery every 10-15 minutes of play time. I know its not temps.
So, last night I set up a new partition and installed Vista Ultimate 64 in an attempt to get playability and now I am able to run crossfire for a little longer before the slideshow begins, but overall the game plays and loads much faster so I am happy except for one thing. As soon as I try to eneable DX10 mode and reboot the game to make the changes, the game goes to infinite black screen.
frustrated. I sense that this is a 4850/Catalyst issue but if anyone has a solution it would be much appreciated.