3.5.0 + LMS + Pi3B+ - syncronisation

I have a strange error exhibited only on the Pi3B+.

I had set up LMS on a Pi3B following the tutorial, and syncronised 3 players - all was working fine.

I then decided to move the set up over to the new Pi3B+ to take advantage of the quicker processing speeds. It works fine but there is an audible stop every 20 seconds or so.

UnSyncing the players cures this and things go back to normal, but the players now are now longer in sync.

For the avoidance of doubt, I repeated the set up on a different SD card with exactly the same result.

Could it be a buffering issue ?

Any and all help will be most welcome...

NWJ