Sub Gen 3 sounds drops out for 5-20 seconds

How are you using the Sub "on it's own" doesn't it have to be connected to a primary Sonos device?

OK...maybe you are using terms interchangeably as what you are posting is confusing. The Sub 3 as you as posted would appear to be in a room by itself especially with the following statement in your post… 

• Even using the Sub on its own exhibits the same behaviour

Please indicate which of the terms below best describes your situation. doing so will allow me to have a better understanding of what you are asking:

• Bonding_The linking of subs and or surrounds to a Sonos soundbar as those components cease to function autonomously and are therefore and extension of the soundbar and receive data from the same.
• Pairing_The merging of two like Sonos speakers to create a wider stereo sound stage as opposed to one speaker that could produce a limited stereo image. Such speakers being the Play 5, Play 3, Play 1, Sonos One, Five, Era 100, Era 300, Move 1 & 2, Roam 1 & 2
• Grouping _The combining (or bringing together) of two Sonos Rooms (Zones using older terminology) to play the same audio in sync

Yes apologies, by 'using it on its own' I mean ungrouped from the stereo pair of Ones in the same room as described above, but still added to a single One (is this 'bonding'? It's not clear) i.e. as far as I'm aware in the simplest possible setup I can use it in.

My point is that all configurations I have tried so far the Sub fails to playback smoothly in the manner described, so it doesn't appear to be specific to the config of the room.

In that case it may be a change in your local radio environment, trying the other two wifi channels may help and is the easiest option.

If that doesn't help waiting for the issue and submitting a diagnostic may be a good option. Submit quickly but you can call in when you have time. Noting through diagnostic number may save a bit of time looking up your information. 

If it were me, I’d say any use of the Sub is ‘bonding’ to a speaker, be it a stereo pair, or a single speaker. It can’t be used by itself (for any others who may run across this thread), it has to be associated with another Sonos speaker.

But that is moot, for your purposes. What may be happening is some sort of wifi interference between the Sub, in its current location, and the device it is ‘bonded to. And my suspicion is more on the Sub being affected, as you’ve tested with different ‘bonded’ speakers.

But there is always the possibility of some sort of hardware issue. If the FAQ linked above doesn’t help, I’d check with Sonos, by submitting a system diagnostic within 10 minutes of experiencing this problem, and calling Sonos Support to discuss it. But don’t post the resulting diagnostic number here, they get sensitive about GDPR.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

Edit: ​@Stanley_4 and I were typing at the same time ;)

I have now submitted a diagnostic and had a call with Sonos Support. They couldn’t spot any issues from the diagnotic data.

At their suggestion I tried moving the SubG3 into a different room and providing a hard-wired ethernet connection to the wired part of my network, bypassing my WiFi mesh network. This fixed the drop out issue with the Sub (!), indicating that behaviour appears to stem either from

• an issue with my WiFi mesh throughput / bandwidth / coverage - none of which appear to be an issue for any other devices used in the same location as the Sub, be they Sonos or otherwise, or
• a shortcoming in the WiFi / networking software or hardware in the Sub Gen 3 that means it’s unable to buffer a new stream sufficiently quickly over either WiFi or SonosNet mesh for smooth playback.

The act of plugging the ethernet in to the Sub automatically switched all my Sonos devices back to SonosNet (a configuration I had previously chosen to undo to see if the behaviour I was having was related to the SonosNet mesh rather than my WiFi). So given that all devices are now on SonosNet and the same issue still stands, this also points away from my WiFi network being a cause, since you’d gather that the SonosNet mesh network must provide enough bandwidth for all other devices to communicate as expected and the diagnostic hasn’t shown the bandwidth or connection to be insufficient. Have also tried other SonosNet channels just in case this makes a difference. It doesn’t.

Please let me know if I’m missing anything. My current conclusion is that the WiFi / networking quality in the device isn’t fit for purpose. Out of 11 Sonos devices in the same system, many of which are much further from WiFi mesh nodes or other Sonos products wrt SonosNet mesh than this one, it’s the only device that shows a problem of any kind.

Next attempt - I swapped the Sub Gen 3 out for a Sub Mini from another room. Result: EXACTLY THE SAME BEHAVIOUR. Sound starts playing from the Mini for first 1-2 seconds, and then drops out for anywhere between 5 and 20 seconds before continuing. So the problem appears to be related to the bonded setup, rather than just being a fault with the Sub itself (although I wouldn’t rule out both subs being substandard, it seems a little less likely).

Currently I think the Sonos system as a whole is totally inadequate for high quality, uninterrupted, music playback - it seems better suited to TV system audio. Given that I’m seamlessly able to playback 4K video + associated audio in any room of my house on my laptop, I wouldn’t have expected such poor network connectivity or processing ability from apparently premium audio gear.