Comment 111 for bug 118745

The remaining cases I've seen of this have been down to the user having an old xorg.conf from either Feisty or an old tribe release. In at least one case, removing the HorizSync and VertRefresh lines in Monitor and rebooting completely resolved it.

Or just run "sudo dpkg-reconfigure xserver-xorg" and it'll regenerate your xorg.conf. For most people you can just accept the defaults for any prompts it gives.