And the real question, is why is this
happening? Some movies are fine and some aren't. It
might be happening when I'm doing a "save as" ...
I think you've got it!
Here, the Save As routine fails to copy the files in the Audio Waveforms folder to the new project. When the new project opens iMovie re-creates one of the waveform files but not the other. Without both files, the waveform is displayed as a thick, purple line.
One "fix" is to close the new project — let's call it Project B — then move a copy of the Audio Waveforms folder from Project A to Project B, replacing the Audio Waveforms folder of Project B. (Option-drag the Audio Waveforms folder from Project A to Project B, which copies it.) Then open Project B.
That seems to work here. Does it for you?
It's strange this problem hasn't been reported before (at least that I recall). If anyone has seen it, please say.
It would be good to report this bug to Apply, Judy. Go here for iMovie feedback:
http://www.apple.com/feedback/imovie.html
Karl