Hmm, never heard of Gordian Knot before. At any rate, just to check all possibilities, you do have all the proper codecs installed, yes? If the videos play then that is an indication everything is fine with video codecs, but if audio does not work then there is an audio codec or two missing. I've never heard of video or audio becoming choppy with the proper codecs installed, or not installed, for that matter. If the proper codecs are not installed, nothing will be seen or heard. As I'm not familiar with Gordian Knot, I have no idea what further compression/conversion utilities it uses to make movies.
No, I like women.