Minim stream being throttled on QNAP NAS ?

[windbag]

Have been using Minim Server on a QNAP NAS to supply mp3 files to BubbleUpnP and then to my old Sonos units (which I’ve used for years). The QNAP is less than a year old and is hosting a total file load of 2.35 TB on a 7 TB total size store.
Things went well until a few weeks ago.

After putting on some more mp3 files into my single-root library on the NAS, a new-to-me Sonos error was being thrown up. To the effect that Sonos wasn’t being fed the stream fast enough.
The effect was to cause a sound file to suddenly cough & splutter, with the rest of the playlist doing the same (seemingly randomly). Diagnosing has been a problem, as it was never the “early” files in an album or playlist that would do this, but a much later track, say track X (10, 15 or whatever).
Re-starting the sequence from spluttering track would then play fine – so it isn’t the actual file that is a problem. But then after X more, it might do much the same – if the playlist/collection was of big enough length.
There have been two Minim server updates since it started, so I’ve re-started diagnosis each time. Sadly, neither have cured the problem.

One thing I tried was to hack out a big chunk of the main library source & forced new rescan via making startupScan property to full, to see if reducing the overall library size helped, but no.
Any ideas why adding a few CD’s worth of mp3s should make Minim to be slower than it used to be ? Or how to work out what the problem is ?
Sonos never threw this error before, even in the old days when the library was on a MUCH older NAS.