Sorry, what I meant was:
1) If you do not have extra codecs installed, then install them.

2) If you already had it installed, try re-installing them, an restarting the n900.

3) Also, you can try uninstalling extra codecs support, and try playing your videos then. If it plays properly then, it's definitely a codec problem.

Chances are the codec's giving problems, and not installed properly. Btw have you tried other media players as well? Do they work properly?