I've noticed a number of concerns regarding movies freezing. I am having this issue with movies in MP4 and MKV format in 1080p and 720p format. I have found that sometimes I can press the fast forward a little and get the movie playing again. Otherwise and more successful I stop the play (frozen), reselect the move, and resume and plays till it freezes again. Seems my issues began with the previous update.
I have made sure the latest server and client apps are the most current available today and still have the problem.
Thecus N5810Pro V2.05.08.btl NAS
Plex Server 220.127.116.11.34-995f1dead (just updated (again))
Samsung Smart TV UN50JU6500
Plex App ver 2.008