Video stutter

Hi everyone

Sorry to bother you again but I have a video issue

I’m streaming from a Synology 1511 NAS as I have been doing for over 5 years, files are 1080p and 4k MKV’s and play more or less perfectly but I have a stutter every 60 secs. Infact I’ve managed to time it twice at 59.4 secs.
Both NAS and vero 4k+ are Ethernet connected into the router as always been.

I’ve had the issue before but for the life of me cannot remember what you advised me last night

many thanks for any help offered