Added it as a radio station, when I listen to it, I hear the music 1 second , 1 second silence, 1 second music, 1 second silence etc etc.
Maybe you can try to stream songs to Sonos for offline playback. Sono supports to add music from the local music library. The upper limit numbers of tracks are 65,000, which sometimes might be less, depending on how large of the track’s metadata. But I think it is enough. Then you can stream local songs to Sonos music library to avoid this error happens.
The next step is to download Spotify as into the local music library. It is unavoidable to use a third-party tool, AudFree Spotify playlist converter to unlock the protection in Spotify songs. Just convert your favourite track to mp3 then add it to Sonos.
When you play music on your phone, you need to establish the connection between the music app and Sonos, then the music will be started on Sonos. Or you could download music and then play them which may be more stable. For example, you could directly download Spotify music or download Spotify music to MP3 via Spotify music converter. Then you could listen to Spotify music on Sono even offline.
Agree with you ,I have used the Tuneskit before,it was good! I also found another Spotify Music Converter from Tunelf,it has same function,recommend ! 
The best way here is to listen to music offline on Sonos. If you are on Spotify, you can download Spotify music offline via DRmare Spotify Downloader, then you can easily play Spotify music on Sonos without any limit.
Another method is to use Allavsoft which can directly convert Spotify to MP3 to successfully play the converted Spotify music in mp3 format on Sonos.
Step 1: Copy and paste the Spotify music link to Allavsoft. Step 2: Click Download button to finish downloading and converting Spotify music offline to MP3.
As I know, it is available to use AudKit Spotify MP3 Converter to download and convert Spotify to MP3 then import the MP3 files to Sonos for listening as instead.
That is probably due to the Internet connection is not stable. I consider downloading the music from Spotify offline as one of the effective solutions. Once you save the downloaded Spotify music by the Spotify music converter, you are no longer interrupted by the bad Internet connection and able to enjoy the music smoothly.
That’s right. I wonder whether the Internet is not stable and I have to play it offline. So I tried to download and play Spotify on Sonos in this way. Then, okay, it’s the Internet. Thanks for that, the DRmare Spotify Downloader works well.
Perhaps you’ve come across the Sonos malfunction problem. In this case, one of the best ways is to download Spotify music to mp3 or other formats for offline listening. Just as the comments above shared, I’ve tried the Spotify downloaders. Surprisingly, they’re efficient! 
The version that I’m using is from Tunelf, you might have a try as well. 