I have a number of videos that I've downloaded over the past few years, and I notice the ones that I have viewed most often are beginning to play in a jerky fashion. The audio seems to work properly, but not the picture. If someone is talking, for example, I'll hear the conversation, but the faces will freeze, and then rush ahead jerkily, and freeze again. Is there any way of fixing this? (I use RealPlayer, if that's relevant.) Anyone know?