Cory, I appreciate the reply and the probing questions. I think the questions may be the key to fixing this, though I don't know that yet. Thanks to you, too, Honestone.
My answers:
Model: iMac (27-inch, late 2013); Processor 3.5 GHz Intel Core i5
OX Version: El Capitan; 10.11.1
3rd party: Yes, I have CleanMyMac3 installed
Virus: No anti-virus software
Browser: Firefox (I seldom use Safari)
Which Apps: I haven't noted this info. It frequently happens when, for instance, opening a Recent file in MS Word. Many of the occurrences are while using an app, not in opening it.
Hard Drive: Capacity: 999 GB; Available: 773 GB
Added question: the only backups to an external device are made by Apple Time Machine; stored on a Seagate GoFlex HD
Thank you for your effort to get at this problem.
rocky63