Windows Movie Maker

The clip you are trying to view may be crupted in some way, or movie maker is lagging slightly. Sometimes movie maker takes a lot of resources so its best not to have to many windows open. When playing video clips, it reads it, and converts it into a media player type file. In this process the clip can slow down, or it would catch a glitch because unlike media player its designed to move along the timeline as it thinks you've added an effect or something when you haven't. Make sure your system is clean use the utility tools, it will help. If nothing else open movie maker after a fresh restart because the memory will be its clearest.