well you have already identified the main problem, your tv doesn’t support dts, so plex has to convert the audio, which your specs appear to be fine for converting audio
make sure subtitles are disabled too.
post a screenshot of your dashboard with a problem video
my guess is your tv is only 100mbs and the video bitrate exceeds the tv bandwidth. and/or you have other network problems.
you could try a wifi connection on your tv (if it has wifi) to see if that improves or makes things worse.
smart tvs apps pretty much suck for 4k.
see also @ Plex, 4k, transcoding, and you - aka the rules of 4k - a FAQ
