I got the same problem some months ago with a radeon 9600, so I installed the ubuntu x.org version.
This didn't solve my problem, so I remove all packages relative to XFree and then I purge it. And then reinstall them.
After that the problem disappeared, I don't know what was the reason but maybe it can help you.
I tried to remove and reinstall X and this didnt help. But reverting to 1009 firmawre fixed the problem.
Jean-Luc
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