Following up on this - it has happened twice in the last several days. Both times I ran pingplotter and went through the process of unplugging cables. Both times it was inconclusive. I thought I had the suspect, unplugged it and everything returned to normal, plugged it in and things went haywire, but then everything cleared up and plugging and unplugging these connections made no difference.

I did get Wireshark running during the second issue.

I did see a lot of these:

[Reassembly error, protocol TCP: New fragment overlaps old data (retransmission?)]

This was between the machine running Wireshark and my main desktop - a Mac Studio. I was running Anydesk on the Mac to connect to the windows PC running Wireshark. So not surprised that things were going wrong between the two when the network was getting overloaded. Presumably this is a symptom of the problem - not the cause?

Most active IPs at the time behind these two machines in order:

Philips Hue hub - unplugged and didn't solve it
A VM running on one of the two machines above. I didn't try shutting that down but I don't think that is the issue.

Is there anything more I can do on pingplotter to help diagnose? Right now I am in wait and see mode until it happens again and hope I can grab more data. Thanks.