Was plex running any streams / background tasks?
Hi Eve, Thank you for your reply and I'm sorry I missed it for so long.
In the interim, I took Plex off the server altogether and the problem remained.
It appears to be a PC issue. The stuttering goes away when I disable MadVR processing. Seems I'm always having an issue with the main theatre PC. It's an i7 3.0GHz with 32 GB of RAM and a Nvidia GeForce RTX 3060 card; MadVR should be more than capable of processing anything I send to it. It's dedicated to the theatre only; the only installed programs are MC, VLC and HomeAssistant Companion. I disable HAC with no effect. Guess it's time for a wipe and reinstalll; been having to do that every six months or so. No clue what Windows is doing that builds up to issues with JR playback. I checked the drive and there doesn't seem to be a need for a defrag; thought maybe JR is caching while playing and drive may be getting fragmented; but it appears fine.
Installed JRMC32 to see if an updgrade helped, but no luck.
If you have any suggestions, I'd be grateful.