Objects drift out of sync with timeline

I'm using Motion 5 to create a photo montage synced to music. I have groups named "Verse 1", "Chorus 1" etc. (This is just an organizational trick.)

The objects are all still photos pulled from my iPhoto gallery - and they slide, fade in/out, rotate, etc. in time with a music track.

The problem is - the farther into the timeline I get, the more my photos are shifting out of sync with the timeline rectangles which represent them. At this point I have pictures which are appear/disappearing 10 frames "earlier" than they should, according to their timeline representation.

For instance, if I place a photo in the timeline so that it appears/starts at frame 8000 and disappears at frame 8025, it'll actually appear at 7090 and disappear at 8015. It does this consistently whether I play at regular speed or step through a frame at a time. I can actually see the playhead positioned 10 frames outside of the photo's range - and yet, the photo is shown in the preview pane.

Weird and frustrating. Has anyone else had this problem? Have an idea how to fix it?