Comment 1 for bug 11148

That's a very general problem. You might want to look and ask around at
http://www.ubuntuforums.org/forumdisplay.php?f=5 to see whether other users
encountered the same problem. It might for example be related to support of your
videocard, see http://www.ubuntulinux.org/wiki/HardwareSupportComponentsVideoCards

If it turns out that it's a new problem, you're of course welcome to report it
here, but please be specific. See
http://www.ubuntulinux.org/support/bugs/document_view for more information.