Movie maker play back troubleshooting

I'm having a problem opening saved movies in the vista movie editor. Smaller
movies are fine but once they get to a larger size, or if there are too many
transitions the movie maker does not want to play back the movie unless I
remove some of the transitions. This does not make sense though since so far
the movies have not been greater than 5.5 MB.
Also, with some of the larger (5.5MB) movies I have problems burning them to
DVD and I get an "unspecified error" message.
Is there a way to fix this problem. I tried replacing some of the picture
files with reduced size pictures but this did not seem to affect the movie
size or its playback significantly.

Please help