I'm using VS2008 winforms over .net 3.5

I'm playing xvid encoded videos using AudioVideoPlayback. Works most of the time OK but sometimes the video freezes for several seconds before resuming at high speed to catch up. This happens on more than one PC (though all PCs are the same model).

It strikes me that the cause could be hard to track down unless someone has seen similar; any suggestions on what could cause this or any suggestions for reliable playing of xvid videos in winforms?