Thank you for your response
Please could you update your profile with its operating systems?
The operating system is Sierra (10.12.1). I've updated my profile on all my devices.
Then, have you had this problem only for a few days or so?
It's been happening for a while -- at least since I installed Sierra and perhaps longer. I'm not sure but it has been 5 or 6 months. It wasn't a problem with Yosemite and not for the first part of El Capitan.
Have you recently updated or upgraded your OS?
Yes, to Sierra
Have you recently introduced a Wacom tablet or pen or other electronic item in the vicinity.
No. There are no active bluetooth devices (other than my mobile phone). The only powered electronic devices are a printer, 2nd computer, a wireless phone and a wifi speaker (not bluetooth). The speaker is new -- the problem existed before the speaker. The wireless phone base station has bluetooth but it is not detectable where my computer is.