I just tried to play the exact same movie as above that I had no problems with earlier, and 1:26 minutes into it I have started buffering.. 3 times in under 1 minute 30. This is on Direct streaming.
I then changed it to Transcoding, 2 buffers within 30 seconds.
Then finally, I changed it to direct playing again, it started buffering at 1:28 minutes in.
Right now, I'm going to restart my PC(the server plex is hosted on) and will report back and see if I still have the same problem.
Edit: After restarting my PC, I have tried the same movie again on Direct streaming, at 44 seconds i start to buffer again.
Now I have switched over to my xbox one to further test this, again via the media player app (DLNA), I don't want to watch the entire movie again but 4 minutes in and I have ran into zero buffering problems, but if my experience is anything like last night, it should work fine through the entire film. The problem is watching movies via the plex app on TV.
My PC can also handle transcoding easily, it's an i5 quadcore and I'm only using 25% CPU while watching it via the xbox one.
You said the plex app does not use DLNA.. If I change my source to Plex rather than going through the app will that use DLNA? It has a different layout and such so I assume so.. Anyways if so, I have just tried it out via TV DLNA to see if the method the plex app uses is causing the problem but even that didn't seem to solve much as it started buffering at 10 seconds and then again at 1:13 minutes
I hope I have given more information so we can get to the bottom of this. My router also stays on over night, so nothing should have changed from last night when it didn't work, until this morning when it did.