I've had this problem for a while and it's driving me nuts. I'm thinking about rolling the driver back a few versions to see if that fixes it but I wanted to check and see if something else was the problem.
Playing videos with a flash player causes the display driver to crash every once in a while. It recovers without rebooting the computer but it has screwed up sketchup a few times and I have lost a good deal of work because of it. Happens with or without sketchup being open. As far as I can tell it's only happening with Flash video.
More about :radeon 4800 driver crashing video play
That doesn't fix the problem it just works around it. I need it to be hardware accelerated so I can work while watching video. Also my CPU isn't keeping up with the video on it's own. It gets choppy at high resolutions. I remember updating the driver and then the problem started.