Dropout on Loop Playback -- WTH?

I had just figured out a great workflow for vocal comping between take layers (thanks to the geniuses in this forum). I chopped up the takes into clips, muted/unmuted each in turn using loop playback. I set the loop start/end by selecting a clip and clicking the "set loop points" button above.

It worked great for a little while. Next time I sit down to work, I get either A) Full dropout as soon as playhead hits the end of the loop, or B) It loops back but playback is slowed down and audio is scratchy/fuzzy.

No system changes in the meantime. Specs in signature.

Suggestions on where to start?

Thanks so much in advance.