AMP fading in and out

Sounds fading in and out

I’ve got 5 Sonos AMPs in my home in separate zones.  Some point this year, I believe since the update, one of my amps/zones consistently won’t play music without routinely fading in and out (to silence).  It does this approximately every 20-30 seconds and lasts about 2 seconds. 

All amps are hard connected to LAN via ethernet from a Ethernet switch. 

It is the same amp every time.  If I play it alone or in a group, it will do this. 

I have powered on and off the entire AV system starting at modem, router and amps. 

This occurs with AirPlay or streaming through app. 

Any ideas what this could be coming from?  
 

I am hoping it isn’t an amp issue as it worked perfectly fine for several years previously.