AE Scrambling Composition

Playback scrambles the composition, which is one long video clip. While seeming to play it through, as the playback head keeps progressing along the time line, the video itself suddenly shows what should be the beginning but in the middle.  In other words, the composition starts showing the beginning again at a certain point, and does this several times, even though the play-back head doesn't go back to the beginning but continues to look as if it's playing it through. I don't know if it is only a playback issue or if it is actually doing this to the composition. I have never had this glitch before, but am using AE for the first time in several months and now it keeps happening.  I have Mac Catalina, OS 10.15.7. 

Thank you!