Same problem here, with actually the same nas (qnap) and the same file (mission imposisbile iso 4k) I have read something about too many audio tracks un the movie can cause this problem but not sure about that, the only way I have found to watch the movie was to move the mouse on the remote and it go again without any problem for other 576 minutes, than again the same problem...
I have just done a preview on another movie, Ant-man and the vasp and again same problem there, it keep on stop the audio, keep going very slowly with the video and then after I clic something it's start again normaly.
It's seems like the player it's going in a sort of low speed render and that cause the problem...