video freezes when i move the slider on the time bar with certain mpeg files

When i play certain mpeg file types and skip forwards or backwards in media player the video stops playing. It plays files from start to finish ok. I’m not sure which mpeg type this happens with but its ok with some, and its ok with all avis. Winamp doesnt display any video at all when i try these files in it. PowerDvd plays them fine and I can skip forward and back with it. I’m using WMPlayer v9. I’m sure I’ve got all my codecs set up properly, I’ve tried various codec packs.

I have a worse problem. Video freezes after the first 5 or 10 seconds. I had this problem in some episodes of south park. Also in Italian Job.