I have noticed that periodically when adding a transition between clips, playback is choppy/ stalled. I have tried deleting and re-adding the clips, deleting and re-adding the transitions, re-importing the thumbnails, moving the footage into a new event, and of course, restarting iMovie. Nothing I try seems to help. I have experimented with several types of transitions; it is crossfade that seems to cause the most noticeable glitch: the end of the first clip will freeze at the very end and then speed up the beginning of that which follows. When I use fade-to-black, the freezing is not quite as apparent but the black screen stays up far longer than it should if functioning normally. I attempted to tweak the transition duration as well but that actually made things worse. To reiterate, it does not happen with all clips but it is rendering otherwise complete projects unfit for viewing. Has anyone ever had this problem? I am currently using iMovie '11 on an iMac running 10.8.2. Any suggestions would be greatly appreciated.