Playback skipping forward and hanging up after starting on measure 21 or later

I click on any measure after #20 and it won't start there. It hangs up for several seconds on one note and then jumps forward 3-6 measures or so. If I start it prior to measure 20 it works fine. Sometimes it plays but not all of the instruments. But now that I'm getting further into my composition it's getting more difficult to play/check what i'm writing, as I have to go back so far. i probably did something wrong to trigger it, but tried undoing several steps after I discovered it and it didn't help. i also tried inserting more measures and that didn't help. I also tried creating a new file and resaving. No joy. Thx for your time!