Well, most likely it is a codec problem. If it is, look at the post that I made in this thread and you should be fine. Just download all the codecs I have there, and if you want, you can download VLC Player too (the link is there, and it's a good player too). If it is not, it could just be the player itself (you might want to consider a different player), or it could be your computer lagging because you are running too many things at once, or you have low RAM. Hope I could help.