Sometimes you might have noticed that when a video is played on Ubuntu 8.10, it is playing with some frames missing and it starts flickering.
Recently this happened to me, I tried different players and tried reinstalling codecs and the VGA driver. But it did not solve the problem. After searching in the web, I found this thread and of course it had the solution I was looking for.
This happens because of Compiz, and you should disable it if you value watching movies over the visual effects. ;-)