Also something to keep in mind, unfortunately nvidia created the tube with less ram (2gb) than the shield pro (3gb).
This means the tube has less ram to help buffer/cache the video as it gets streamed from the server.
While watching the plex dashboard, also keep an eye on the bandwidth graph, if you see a small drop, then large spike in bandwidth during any stuttering or pauses/buffering, then most likely there is nothing wrong, other than the tube itself is not able to handle the high bit rate of the video.
If you see the server is transcoding the video or audio, then you may want to try to identify why it is doing so, and try to alleviate what is causing it.
As FordGuy mentioned, how/where the shield is plugged into can matter, best is to connect shield to the AVR or soundbar directly, instead of the tv.
this huge thread @ [INFO] Plex, 4k, transcoding, and you - aka the rules of 4k may be helpful explain the various issues with 4k streaming, the thread is very long, but worth reading to see all the different points of view and opinions.
