Tracker (vertical) Line on Screen Drifting Behind During Playback

Song sounds fine and plays in-time but, by bar 25-30, the vertical line lag is noticeable; by bar 60, it's one quarter note behind the beat.
 
* I have a few audio tracks (small sound effects) in the file, but the lag remained even when they were removed.  (Pretty sure this all started after I dropped in--then stretched--the audio when I was first teaching myself the DAW.)
 
* From this midi-only file, I exported as a MID1 file then imported back into SONAR as a new file.
Happily, the indicator stayed in time when first played--all the way to the end.  But then it drifted behind on subsequent playbacks.
 
Any ideas?