Audio is fine, but video is frozen!

My movie/project plays fine in WMM. When I create a movie file (~140Mg), the
movie plays, but at some point the video freezes on a picture, while the
audio continues just fine until the movie ends.

I have some half speed "video effects" in the movie. Any ideas greatly
appreciated.
Using WMM 2.1 on Windows XP with 128M RAM.