Thank you for a thoughtful response but it is not exactly the problem. I am not losing the emails. They remain in the list with to/from and subject lines. It is the Contents of the email that disappear temporarily - maybe for ten minutes or so. The content is blank and suddenly reappears. In all mailboxes: Inbox, Sent, Trash etc.
The email is still there and suddenly the text of the email itself disappears after having been visible for a while. The emails remain visible in the list but the text and attachments disappear and reappear after a few minutes. Double clicking opens the email but there is no content, no text - for a while. Minutes.
I think they are still on the provider server and are just not accessible at all times.
But it is the content of the message that disappears, not the message itself.