I’m re-posting this because, apparently, there’s still someone who’s looking for this post. So here it is, from the original post I made some years ago.
After an upgrade I had issues with tearing and waves corrupting the desktop on my external monitor (no issues on my laptop screen). I decided to format and reinstall everything. It solved a tons of other small issues but not this one. External monitors (LCD or CRT) won’t work with Ubuntu 10.04 notebooks with Radeon cards (I have a Mobility radeon x1600). You’ll see a distorted image. Waving like the refresh rate is incorrect.
SOLUTION
I just did the following (on a bare install of Ubuntu 10.04)
Fire up the terminal and type
gksudo gedit /etc/modprobe.d/radeon-kms.confa blank file will open up. Type in the following.
options radeon modeset=0Save, exit and reboot.
Beware that this solved my particular issue but it may not solve your own. My issue had to do with waves and refresh problems on an external monitor connected to a Notebook’s Ati Mobility Radeon x1600, running Ubuntu 10.04. That’s it. Your mileage may vary.