Windows Movie Maker help?

I am making a video that needs to switch between two different people. I achieve this by synchronizing the sound (Lip-synch video) with one part and then cutting/pasting the other part into the gap where the other person talks. Recently, however, whenever I try to access part of the video with the other person (I'm playing both of these roles) through the timeline WMM always freezes and won't let me play ANY part of the video. I was able to get it to work again by undoing an action, but recently that has stopped working as well. The problem only seems to be with one part of the video, though, the second person.