Squeebox Pauses every 10 seconds

I'm running LMS 7.8.0 and I have 6 raspberry pi's running Max2Play. About every 10-30 seconds the music will stop for about 10 seconds, then restart. If I reboot the router and then add one raspberry pi, then it does OK. I am able to add devices, up until about 4 devices, then the stability decreases.

I have a wrt1900AC. The server is connected via an ethernet port. The raspberry pis are all connected via 802.11n wifi, except for one which is hardwired.

Any thoughts?