Dear Sam,
I know this is an old thread but…
In February this year, I received 2x Vero 4k+, which replaced my older Vero 2.
I noticed the stuttering as well on the new devices, which was not the case with the Vero 2.
I decided to wait for the kodi 18 update, hoping this would solve the issue, which was not the case.
Adjust Refresh Rate enabled or sync frame rate did not help.
My media is stored on portable hard drives, Buffering is not an issue I think because it may happen with media files of smaller bit rate than those where it does not happen (8600 kb/s).
I managed to capture the stuttering with my smart phone, not sure to where i should upload for you (if you want to see this).
Hopefully this can be fixed.
[edit]
Solved here: Stuttering video 2 - #4 by bmillham