On One Song, the On-screen Bar/Beat Indicator Slipping Behind

Playback is fine on one song, but the beat indicator line begins to slip after about 20 bars; by end of song 5 min. later, it's a full quarter note behind.
 
All midi except for a couple of small audio clips inserted (bell tree, etc) and, as I recall, this may have been when it first happened.  Tech Support suggested copying all all data into a new file, to no avail.
 
Any thoughts, anyone?