There' s a lot of information on this in this forum, but basically the Media Player is a weak part of the N900. I would try to install and use Mussorsky (extras devel or testing?) to enter the mediafiles and tags and look for irregular filenames. And I would install Mediabox to see what happens. On this forum you can find ways to kill the media informationlist used by the mediaplayer. Than it generates a new file. Never tried that. Search under applications.
EDIT And in the browser, MicroB delete history (keep cookies etc). This in some strange way makes the system more healthy.
You do have the latest firmware 1.1? I only had problems before.