Thanks for the comment.

But what causes Excel to get all "hung up"?

How can you know it has happened? (Without checking each time you you change
anything that it has worked - in which case why have Excel at all!)

Is there any way of preventing it happening?

As for "clearing" it, I expect that finding and replacing all "="s with "="s
across all worksheets in all open workbook, while clever, will be slower than
Ctrl-Alt F9, which seems to work. Will it also cover hidden sheets/books?