Comment 80 for bug 161893

Based on the description of the symptoms, it sort of sounds like this might be caused by an incorrect AGPMode setting, which is relevant when DRI is enabled. You should test if changing this makes the issue go away. In your /etc/X11/xorg.conf set AGPMode to various values like this:

 Section "Device"
      ...
      Option "AGPMode" "2"
 EndSection

Possible values include 1, 2, 4, 8. You can see what it's currently set to by looking in /var/log/Xorg.0.log. It's worthwhile to test all four values even once you find one that works. If this takes care of it for you, we may be able to establish a quirk to set it for your hardware combo

For more details on this, please see the "ATI AGP Mode Quirk" section at https://wiki.ubuntu.com/X/Quirks