Ah... I found a fix for it. Apparently this error is fairly common with nvidia cards.
http://ubuntuforums.org/showthread.php? ... tching+glx
the trick, apparently, is to run nvidia-xconfig.
On my computer, I fixed it this way:
sudo mv /etc/X11/xorg.conf /etc/X11/xorg.conf.backup
sudo nvidia-xconfig
sudo mv /etc/X11/XF86Config /etc/X11/xorg.conf
That would have worked as is, if I had a CRT monitor. Instead, I have an lcd monitor, so I had to change this line:
VertRefresh 50.0 - 150.0
to:
VertRefresh 50.0 - 75.0
and this line:
Modes "1600x1200" "1280x1024" "1024x768" "800x600" "640x480"
to this:
Modes "1280x1024" "1024x768" "800x600" "640x480"
now it seems to work perfectly.
Hope this helps anyone else!