Hi Mark,
It is likely that Paul and Doug are right that it is document corruption.
If you are using macros it is possible that your code is actually creating to many levels of fields. To diagnose that, we would need to actually see the code, or at least I would.
If the document corruption cure works, please mark their responses as answers so that they will come to the attention of others who have the same problem.
If you want, save a sample document with the problem on OneDrive or Dropbox and paste a view link here. Remove any confidential or sensitive information first. I or someone else can take a look at it.
For OneDrive: See https://support.office.com/article/share-onedri...
For DropBox: See https://help.dropbox.com/files-folders/share/vi...
This forum is a user-to-user support forum. I am a fellow user.
I hope this information helps.
Please let me know if you have any more questions or require further help.
You can ask for more help by replying to this post (Reply button below).
Regards