I suspect the Sonos software looks for the lowest latency connection. If the ping time through your router via Ethernet is greater than via WiFi, my guess is it’s switching from one to the other.

I’d probably first try a network refresh, by unplugging all Sonos devices from power, the rebooting my router. Give the router a couple of minutes to recover, then plug back in the Sonos devices. Give them a couple of minutes to come back up, then check for updates (there was a iOS update, at least, today) and apply it.

Finally, if that doesn’t make any difference, I’d turn off the radio/wifi on any wired Sonos devices, using settings in the controller. 
 

I suppose that wasn’t ‘finally’. If none of that helps, I’d submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it. Don’t post the resulting diagnostic number here, they get sensitive about GDPR.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

Could be the cable connection becoming iffy, unplug and replug both ends several times to clean the connection points.

As above, only support can see the internal logs to see why it is switching, we users can only guess.

I’ve swapped the cable and port on the router, so I can rule that out. Unless it’s a faulty port on the Five.

I can see the behavour from my router logs. The router detects BOTH MAC addresses on WiFi simultaneously which should not happen. Both MAC address completing WiFi handshakes every 10 minutes.

This is the DHCP conflict my diagnostics detected:-- 
- WiFi MAC tries to get IP → Rejected (twice)
- Ethernet MAC succeeds → Gets IP
- Multiple rapid DHCP requests (device is confused)

I’ve had a 2 hour call with support, unfortunatly they could see it happening, but really had no idea. I’ve raised it up the chain within Sonos, so hopefully they can get to the bottom of it.