Mac 10.13.6 16g RAM, Finale 26.1 / 25.5

Here's what's happening:

On playback, a file will play fine for several measures and then the bottom voice will drop out, and stay out. The same file will play back correctly at other times, so it's not a file issue, and it happens with many files.  These are generally unaccompanied vocal trios (the project I'm working on right now), so it's not a memory issue, and I've got plenty of memory. I've tried running Finale with no other applications running, and it seems to make no difference. There was nothing that I added where I could pinpoint, "Oh, it started misbehaving after this." This has been happening intermittently for some months under 25.5, and I could generally just play back again from the beginning and it would be fine, but it seems to have gotten worse, where it would continue to malfunction on successive plays. Last night I bit the bullet and upgraded to v. 26, figuring that surely the bug would have been fixed, but it's still happening (and yes, I restarted the computer after installation). Having spent $156 for no solution, I'm a little miffed. This also happens when saving audio files. Perhaps it's some odd Audio Units issue. All I can think to do at this point is to upgrade the system to 10.14, and I'm hesitant to do that. Does anyone have any ideas?