Did you try reverting to player 8.6.0.20351?
It will most likely fix your issues.
Download and install player version 8.6.0.20351 from here.
Once installed goto the player advanced settings. Allow HDMI Audio Passthrough, Then Also under Advanced. Change Enable New Video Player to OFF. Also set the shield to NOT update plex automatically.
Hope this helps, I dealt with these problems for over a year trying everything until I found this. Plays perfectly with this version.
