Movie maker glitching

I'm making about a 20 minute long film with photographs in movie maker. Everything started out working fine. I've been adding scenes and once they're in I'll test play them once I have the time duration for each photo set. When I do this everything seems to be working. I added scenes until I've gotten to around 15 minutes with just over 1100 photos. I decided to try playing the film from the beginning to see how well it flows. Every time I do this, about 6-8 min in, the film starts to slow and photos pause as if it's having trouble loading. It will do this for a while until it can't handle it anymore and it just freezes, sometimes going to a black screen. Any ideas what could be wrong? I thought maybe my photos were too large so I made the MB smaller for some that I knew were really large files (only about 150). This didn't seem to help. When I played it from the beginning it started glitching about 3 minutes in. I could really use some help on this. It's a project for school that is due next week.