It’s hard to make a great determination with the information you’ve provided. I would lean towards the issue being with Spotify’s servers, but there’s also a possibility of a DNS issue, and even other more rare issues. 

I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it.

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 phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.

Same issue. Skips and stops with Spotify 

I think there is a Sonos and Spotify network issue. 

So, what did Sonos say, when you called them?

There is one Sonos ‘player’ software, if there’s an issue with that, wouldn’t it be across all streams, and not just Spotify? Additionally, all Sonos does is point that ‘player’ software to get a stream from a server provided by Spotify, there’s zero Sonos ‘server’ connection involved. 

There are a litany of possibilities between the ‘player’ on the Sonos speaker, and the server provided by Spotify. Networking is hard. This is why in cases like this there is often the recommendation to call Sonos Support directly to discuss it. Only they have access to a systems diagnostic, which is the most likely place to see what particular error is being recorded.