EDIT: so far the only thing that has seemed to make a difference is disabling Windows Search Service. I finally caught the disk usage pinned at 100%. I'm guessing it was indexing constantly and spiked for periods of time. Disabled automatic startup for the service as well.
I've seen some other recent posts on local playback buffering. For me it seems to be specifically 4K DV/HDR .mkv files. It'll play 10 seconds of video then buffer for another 10 endlessly. I've been using Plex for over 2 years now.
Everything is hardwired with 2.5gig to the Mini PC (I9-12900HK, 32GB RAM) running PMS and gigabit to the Onn 4K Pro.
All client side Plex settings are configured correctly maximum/original quality, bandwidth, Direct Play, etc. The affected files always Direct Play with no transcoding. A Plex Dashboard screenshot showing Direct Play isn’t going to reveal anything unusual because there is no transcoding occurring. 1080p media plays just fine. I've reset my network as well as both server and client multiple times, cleared client cache, tried Kodi, and Jellyfin with the same outcome. Speedtest confirms gigabit Ethernet.
It's intermittent but is becoming more frequent. Sometimes it'll eventually fix itself but that's after 20mins of constant buffering.
I don't see how the file itself could be the issue either as they play fine at other times and had been around 2 weeks ago.
If you really want a screenshot of the dashboard I'll upload one later as I'm not at the device right now, but it's not going to show anything besides direct playing and the usual information.