Kind of the same trouble here, random hard freezes requiring shutdown...
Tested extensively HD, Ram, and downgrading solved the problem eventually.
HD4850 on Slackware 12.2, catalyst official package.
Switched back to 8.12 since it was the last one I used regularly, after an unsuccessful attempt with 9.2 (same freezes).
Makes me regret to have bought from ATI once again (...) since I burnt my previous card trying to fit it with the Musashi Scythe in my crippled SP35P2 barebone...
Whatever, I'll wait for the next one to get a nice GoogleEarth output.
It looked promising.
Any ideas on where these freezes might come from ?