Are any of the speakers wired with an ethernet cable?

Yes - the Beam are wired.

Make sure you keep WiFi enabled on all speakers in the Sonos app including the Beam.

I’d suggest, since you have a signal, but it is occasionally interrupted, that you’re suffering from either wifi interference , or possibly a duplicate IP address issue caused by your router. Beyond the information in the linked FAQ, I’d also suggest a network refresh, by unplugging all Sonos devices from power, then rebooting your router. Give the router a couple of minutes to boot back up before plugging back in your Sonos. Of course, you probably should wait another couple of minutes to allow your Sonos devices to boot up, and reconnect before testing. 

It’s important to remember that when Sonos is used in a surround mode, the surround speakers connect to the soundbar via a hidden 5Ghz channel, which is why ​@GuitarSuperstar was suggesting the Beam may have had its WiFi turned off. The surround speakers do, however, receive IP addresses from your router, just as proxies through the soundbar. So if they’re ‘fighting’ for an IP address, they will drop. Could be a very short time, or a very long time, depends on what they’re ‘fighting’ with. If you’re capable (my router doesn’t allow it, but most do), setting up reserved IP addresses in the router is best. Instructions on how to do so vary, but should be covered in your router’s manual, but the above suggestion is a good ‘temporary’ fix, and would work until the router messed up again. Potentially permanently…potentially it would fail the next time the speakers reboot, like when Sonos updates the firmware. Or, like me, you have a power outage. 

It seems that the problem was solved by activating Wi-Fi on the Beam.
Thanks for the help!