Running one of these clips for 2-3 minutes the CPU would go anywhere from 6% to 78%. I noticed that when the rate was on the way down, it would level off at around 10% - 15% and I would get a change in the still image. When that change occurred, it always seemed to be at a key frame (to use the parlance of Virtual Dub) - changes in the image brought about by switching from one person to another in a dialogue exchange. Once the change occurred, the CPU would drop to 6% and then work its way back up.

Thank you for taking the time to reply.