@TimothyLambert Also considering going to Settings>Transmission>NAT>ALG. Disable all the ALG's and retest.

@TimothyLambert The best way to look at it is... Click on the client list and click on the 3 dots on the right. Check customized columns, check signal, wifi connection, rx, and tx. This will show you...

@TimothyLambert 2 things here... What DNS Servers are you using? Use CloudFare or Google DNS. Also by chance, do you know the signal strength of the clients that are having issues?

@TonL Have you tried a new cable between the cable modem and the WAN port of the router?

@TonL What are your wan settings? DHCP, PPPOE?

@lukass2000 Sonos can cause a multicast storm that floods the switch... that in turns fires off STP. I Would try to turn down RSTP to STP and disable loop detection. Sometimes RAPID fires off too...