I used the command-line utility xrandr to fix the problem. Running the tool with no arguments prints a list of displays and available display modes for each. This is handy since you need the name assigned to your display by your OS for the next step. Next, use the “–newmode” option with xrandr and specify the modeline which describes the display configuration you wish to use. This modeline generator might help you create the modeline you need. Once you create the new mode, use the “–addmode” option to add it to the list of modes supported by your monitor.

Finally, add this command to your ~/.xprofile file (or something similar) so that when you start your machine the new mode is automatically added and available (this way Ubuntu automatically reselects it too). This is what I ended up adding to my ~/.xprofile file:

xrandr --newmode "1920x1200_50Hz" 128,300 1920 1968 2000 2079 1200 1203 1209 1234 +hsync -vsync
xrandr --addmode HDMI-0 "1920x1200_50Hz"

Note: If you set your refresh rate too high, your monitor will probably flicker occasionally. If this happens, try lowering the refresh rate by lowering the pixel clock value (the first number in the modeline).

Update to the latest ATI driver and Catalyst Control Center (optional) – This should be straightforward if you own a desktop, but on my Dell laptop ATI’s installers won’t upgrade your drivers (or even install new ones after you’ve uninstalled the old ones — apparently only the OEM drivers will install). Anyway, Ruud Ketelaars has developed “Mobility Modder” which can tweak the installation files so that you can upgrade your laptop’s ATI drivers. You can find his software and instructions here (scroll down).

Add the max resolution of your monitor to the available options:

1. Open ATI’s Catalyst Control Center (CCC) by right-clicking on your desktop and clicking on the “Catalyst Control Center” option.
2. Make sure you are in advanced mode (lots of menus on the left side).
3. One of the menu options on the left side will be something like “Digital Panel (DVI) 2″ and will probably be listed just under “Notebook Panel Properties 0″ if you are using a laptop. Click on the “HDTV Support” submenu of the “Digital Panel” menu.
4. Under “HDTV Support”, click the checkbox next to the resolution and refresh rate which you want to enable for your monitor. For my monitor, this was “1920×1080p50″ (50Hz).
5. Click “Ok.”
6. Open Display Settings like usual and you’ll be able to choose the new resolutions you enabled (right-click on the desktop, choose Personalize, then Display Settings)