iMovie 9 keeps crashing during editing

OS 10.6.8 intel

iMovie 9

I'm using iMovie to make a movie. I've been regularly importing events into the project screen and editing them for length.

This is going to be a black-and-white silent movie, so each clip has the color taken out (via zero saturation), the audio reduced to zero, and the video sped up to 200%.

Things have been going relatively okay, but the video is now up to about 15 minutes long, and as I'm working on the project, every 10-15 minutes or so, I get the spinning pinwheel of doom (accompanied by that soft computer rapid "ticking" sound) and then iMovie disappears. Within 10 seconds I get a message saying that iMovie has quit unexpectedly, and I choose the Reopen option. Within a minute or so, iMovie is open again and everything looks okay. I then am able to happily work on my project again until the same thing happens in another 15 minutes or so.

I've made plenty of movies on this Mac, and this has never happened to me before. I'm wondering if I can do something to prevent it. I've got 34 gigs of space on the HD, and the movie currently is taking up 45 MBs.

My first thought is that with all of the altering of the clips (sped up, b&w, zero sound), the computer has to do a lot of calculating and remembering, which can create a heavy workload. Perhaps if I went slower, or if I took a break every 10 minutes it might make things easier for it.

Just a hunch, though--anybody have other thoughts for me? Thanks.