What player are you using? It might be a codec issue. For codecs see: http://www.free-codecs.com/ OR http://www.divx-digest.com/software/nimo_pack.html

Not sure if GSpot would be helpful here but it can identify which codecs you need: http://www.headbands.com/gspot/

But yes it could be something to do with the way it was encoded. It could have been a poor quality encoder or encoded at too high or low a bit-rate. Your comp might be struggling to handle it.