Well, it really depends on the movie info. I think that it is either caused by a lower frame rate of the movie, or that your computer is dropping frames. If you go into Quicktime and use Get Info (Apple + I), you should be able to get the frames per second (fps) and playing fps (they should be either ~24 or ~30). If you could also post the codecs and resolution, it would be helpful, it could be hardware limitations or the fact that OSX doesn't like some codecs.