Hello everyone and thank you for all the ideas. The problem did in fact appear in Lectora. I think I have solved it (not quite sure yet) and apologize to you for all of your attention to this issue.
The original layout created by the question editor was modified by one of the people working on this title. In making the changes they accidentally made the question field partially overlap with the entry field. When I noticed this, I went through all the questions, making sure that no overlap happened. The problem *appears* to have gone away. I say “appears” because the error seemed to appear randomly whereas you would expect the error I described to be systematic. Perhaps it depends on the point at which one clicks in the entry field…
Anyway, I really do want to apologize to you all who have been trying to figure out the error. I will write again if the error returns but, until then, we can probably let the matter rest.
Thank you so much