Originally Posted by Dizzle7677
Set the Powermizer control to 'prefer maximum performance' instead of 'adaptive'. Or if you're using a pre-release of some software like Firefox 3.6.2pre it might cause troubles. But you never really know what the cause is until you debug X. See if that helps.
More info: https://bugs.freedesktop.org/show_bug.cgi?id=15473
This looks like it works, hasn't locked for me since I tried that.