You problem sounds like you do not have enough power to decode files. How much memory do you have?
But mainly it depends on processor. Yours is, certainly, outdated, by I have 2500+ athlon and can play almost all mkv files succesfully.
So yeah, try to clear the background programs that might be eating your processor power. If you run a torrent, archiving some files, run antivirus and copying large files all at the same time you would have a trouble watching video. Or it could be a virus slowing you down.