iMovie HD 6.0.3 playback problem
I have this same problem with transitions. They render the playback fuzzy whenever a transition occurs (in my case, cross dissolve).
Not only that, but suddenly playback is not registering the correct duration of clips for still images. For instance, it will play a clip set for 3 seconds for 5 seconds, eating into the duration set for the next clip. That clip will then only register for a second or so.
Infuriating.