ahoward, I understand that you can't reproduce this and if you can't reproduce you can't fix. It happened to me again and I'm going to give you one more set of data. Maybe it will be helpful if it happens to someone else and they can give more info.
According to Activity Monitor it does not appear that PM is using undo amounts of resources or that the computer is being more than "normally" heavily taxed.
I believe I have the recommended Cache settings in my PM preferences. Please let me know if I should adjust them.
I had been using PM for a number of hours today, however, mostly for adding Metadata and only somewhat for comparing and deleting images. There were 400 images in the trash.
This time I noticed PM lagging when moving quickly between images, and that is when I looked to see if the rating in the right image comparison had disappeared again (which it had).
Best regards,
Ric Cohn