You’re not alone - we all got the same problems but no response from any developer yet.
Here: Buffering w/ 10mbps Direct Play video. WHY?! - #3 by Maddeen
And here: Playback freezing. Help with log files - #9 by Maddeen
And here: Impossible to watch anything - #3 by Maddeen
and last but not least - here: Extreme Buffering after 10-20 Minutes of normal playback - #8 by Maddeen
Not sure whats or even if anything happening behind the Plex curtains. But I don’t like that “no reply attitude” 
