Windows Movie Maker is freeing up?why?

So i'm making a speed paint video and they're two separate videos , so i sped one up and the other one too, i added music, and i watched it just to see how it turned out, The first part i recorded and sped up was fine, but the second part started to freeze up and the music would freeze up with it too, i've made lots of these before but i don't know why it's doing it now? please help