I think this is now solved. I logged into the modem dashboard and enabled “Bridge” mode - this turns off the wifi that comes with the modem. (I am running an Eero mesh system for my wifi). Behavior has returned to stable and the gremlins appear to have disappeared. For Xfinity customers:
Toggle Bridge Mode on and off
- Using a device connected to your home network, open a web browser and go to the Admin Tool at http://10.0.0.1.
- If you’re disabling bridge mode, this device must be connected to your gateway with an Ethernet cable.
- Enter the default username and default password:
- Username: admin
- Password: password (case sensitive)
Then enable Bridge mode.