First, do you have a BRIDGE from Sonos? If so, try wiring your speaker directly to the router (at least temporarily) and remove the BRIDGE from your system. It’s know to have a funky power supply, those fluctuations make it hard to track down any specific issues. Ultimately, you could buy a replacement power supply, or a Sonos BOOST (much newer tech), or just leave a Sonos speaker wired to your router. 
 

I’m assuming you followed the instructions in the new router - installation instructions FAQ?