@Demolition

Clock speed does seem to have influence on playback. I use QCPUfreq and overclock on a per session basis (max @900), usually when I want to watch a movie from the phone, displayed on the TV. I only utilize mplayer from the command line, though, as my experiences with both the default media player as well as KMPlayer were not very good, especially when I wants to use (external) subtitles.

That being said, however, I have ran in to some files that had trouble playing. What I did find is that ffmpeg does a pretty good job converting these "unplayable" files to something that actually does work. An example for converting a .flv to mpg can be found at the link below:
http://www.ubuntugeek.com/convert-fl...ng-ffmpeg.html

If you insist on using the default media player, converting on your n900 might be helpfull with that.