Two things come to my mind: dynamic spell check is on, preflight is on.
Depending on the power of your computer, this can take a lot of time to finish every check.
As soon as I saw the reply, I thought this was it and would certainly work!
I turned off Dynamic Spelling and preflight, closed down and trashed preferences and restarted .
Still doing jittery hourglass.
I have saved as idml and then opened again in InDesign and resaved as .indd, but no changes.
It is only happening on this one document.
Thanks for the suggestion.
OK, a third idea: Indesign gets memory management problems if you link too many text frames. Every letter you then type is followed by a symptom very similar to your description. This can be solved by splitting the frames again with the SplitStory script.
Thanks again, but I don't have any linked text.
But I have solved the problem with your previous advice.
I thought I had pre-flight off but I did not.
It was a silly oversight on my part.
Thank you for your help and support.