https://bugs.freedesktop.org/show_bug.cgi?id=28426
--- Comment #16 from Michel Dänzer michel@daenzer.net 2012-01-30 07:41:01 PST --- (In reply to comment #14)
It happens randomly and usualy goes away after a while.
Given that this bug really seems fixed for others, and comment #7 says only a reboot helped, your problem sounds more similar to bug 33183 than this one.