Comment 27 for bug 154104

My experience, and discussion on the Ubuntu forum, suggests that this is an issue with the upgrade from Feisty to Gutsy. New Gutsy installations didn't have the problem.

Changes to xorg.conf were all in vain (and all the helpful comments from people who had done fresh installs of Gutsy were worse than useless!)

I initially fixed it by following the suggestion on the ubuntu forum to:

apt-get remove .*mesa.* .*xserver-xorg.*
apt-get install ubuntu-desktop

But I ended up doing a fresh install of Gutsy.