Comment 5 for bug 42652

This is a known issue - the ubuntu xorg setup scripts mis-detect monitors and then just use a generic default for the monitor's h sync and v refresh rates.

The work around is to change these numbers in /etc/X11/xorg.conf to the correct ones for your monitor. In this case:

  HorizSync 43.89-48.51
  VertRefresh 60

If that doesn't work, if you're using Feisty you could also try simply moving xorg.conf aside and restarting X, and see if it auto-detects the settings properly. (I'd be very interested to hear either way.

In any case, this seems to be a dupe of 3731 - marking as such.