WiFi on Beam shouldn't be disabled. If so, there's no direct connection between the Beam and your surrounds and it needs to travel thru your LAN, then to you WLAN and then to the surrounds. This adds up to latency and that's why your surrounds are occasionally dropping.
Best option is to turn the WiFi back on Beam. It will allow the Beam connect to the surrounds thru a dedicated 5GHz network. At the same time, it'll create SonosNet. If you wan't to disable SonosNet, unplug the Ethernet from the Beam.
Other option is to wire the Beam AND the surrounds. Try avoiding hops, so if possible connect the Beam and the surrounds directly to the same switch / gateway / router.