Well, it has been over 4 days since I added a new switch between the WAN port and the ISP equipment, so that I could also add a second device with an IP address to that switch to try pinging in case of an outage.
Fortunately(?) the system has not gone down since making this change. I am starting to think that the problem may have been with the hardware. I had a different cheap switch in place before, which may have been causing the problem.
I'll keep watching…