I have tested some of my various SD card backups with the following results -
Current system: OSMC = 2018.07-1; Kernel = 4.14.34-4-osmc; Dropouts = Yes
20/3/18 backup: OSMC = 2018.03-2; Kernel = 4.14.26-2-osmc; Dropouts = Yes
24/1/18 backup: OSMC = 2017.12-1; Kernel = 4.9.29-13-osmc; Dropouts = No
06/2/18 backup: OSMC = 2017.18-1; Kernel = 4.9.29-17-osmc; Dropouts = No
This confirms james4’s suggestion that the problem arose in March 2018 and indicates that it is likely to be related to the change of kernel from 2.9 to 2.14, as suggested by Ronnick in post 11. Therefore, I will try downgrading the kernel next week, following Sam’s instructions in this thread Playback Video - sound stuttering after March 5th Update, which is presumably a slightly different manifestation of the same underlying cause. Whilst, as Ronnick points out, this is not a good solution, until a proper fix is available I think it is preferable to have an out of date system that works properly than an up to date on that doesn’t.
PS Since I have these backups, I could relatively easily provide logs under identical conditions for a working and a non-working system, if this would help the developers resolve the issue. However, since a working fix has previously been provided on both these threads and then reverted, I suspect they probably know what the problem is, just not how to resolve it without causing other problems elsewhere. Is this correct?