Timing slide advancement in iMovie

I am creating a slide show of photos in iMovie HD (not iMovie 2009). All clips have been set to display for 3 seconds, yet when I view the slideshow, some play for longer and some play for a shorter time. The slides don’t advance when the playhead seems to indicate they should advance. Most of them advance instead when the playhead reaches the end or the middle of the NEXT slide, and there’s no setting that I can find that would explain this — as I said, they are all set to display for three seconds. Any suggestions? I’ve worked around this in the past by adjusting the display time of individual slides, but that’s a tedious fix… Thanks!
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Michonne