Problems playing 2 synced SBTs at the same time - new problem just recently.

I have two SBT units both synced and using my Raspberry piCoreplayer with frequently updated LMS 7.9.1. Playing an MP3 album from my media library on the server works correctly until I turn on the second player, then the music pauses every 30 seconds or so before continuing. After a while they skip to the last track in the playlist, and plays that. Then DSTM kicks in and another 25 tracks are added but the players pause and then skip to the last of the newly added files. I have only noticed this since about Thursday. Can not think of anything that I have changed at that time. Has been happening just now but there are no new entries in LMS server log.

Cheers, David