Vista - playback problem in Movie Maker

hi
I have Vista Home premium and mm 6
i have been working on a project for weeks and all of a sudden when i play
it back the video jumps and eventually stops though the audio keeps playing.
I have currently 5 projects on my computer 3 will work fine no problems the
other 2 are the most recent ones. They are the same virtually but with extra
bits i put in. these both do the same thing. The video files themselves on
their own play fine but when playing back the whole project it doesnt work. I
have tried copying and pasting but it doesnt work
Any tips? I dont want to restart it all as it took way too long.
kel_the_ghostbuster@xxxxxx
Thanks