Following on my last thought, I began removing images to see if I could get down to a stable content level. After removing one panorama from the project, I seem to have achieved this goal. Just for fun, I then put this image back into the project so content wise it was back to where I started. Surprisingly, the problem did not return.
It seems that the program wrote some faulty script which it was forced to rewrite when I eliminated an image.
Anyway, if others run into this problem, there is a strategy to try.