OpenPHT video freezes in random

Ubuntu 16.04
CPU: model name : Intel® Core™ i5 CPU M 520 @ 2.40GHz
RAM: 5897080 total

Run openpht on same server as plexmediaserver
Got freeze when play any type of video, any time, any way. Nothing in log debug.
Tried any settings. What can i do to prevent this?

And it’s freeze only video, not audio playback. After second two it runs faster until video get synchronize with audio

well, i guess i fix problem.
unity with compiz makes video in full screen little laggy(guess this because composite plugin, but not sure)

anyway, gnome-flashback works fine, without any video freeze