TLDR: Boost was the problem.

I've got a 6 component + boost Sonos System operating in 3200 SQ ft  over 3 floors.  For years, whe streaming TuneIn or any other service the system has performed flawlessly whether playing in a single zone or grouping all rooms together  BUT when playing my library on a Nas wired to my router I got constant stuttering, drop outs and track skips to the point that I just gave up on playing my own Library (5000 CDs ripped flac).

I've also got a 4 Node Mesh Network so this morning I just had the thought that there must be enough bandwidth in my network to support the Sonos and the Boost with Sonos Net might be the problem.   Long story short, I went into Sonos settings, enabled my wifi network, then disconnected the boost…..

I've listened to 3 whole albums perfectly synced across all rooms without a single stutter or drop. 

SonosNet isn't always the best solution.  I had myself convinced that having Sonos run on its own dedicated network would provide the best performance.  Not so, in this case SonosNet was creating wifi interference with my mesh network which has the stronger signal.