Need to use one of the 2 custom firmwares for ROC-RK3399-PC-PLUS in this forum, posted by me.
Kodi
To enable auto frame-rate switching Go to Settings - Video Player Enable Adjust display refresh rate and set it to 'On Start/Stop'
In Settings - System - Display - Whitelist: Enable the resolutions supported by your display eg. 1080p tv
Download Link (Kodi is already included in my firmwares)
Frame-rate switching allows you to have a smoother video playback experience. It's very noticable on a 4K tv and when playing 23.976, 24, 24, 29.94hz video content that always play at 60hz without frame-rate switching. Then it causes micro video stutters, camera zooming from left to right not smooth and audio becoming out of sync after a couple of minutes. Frame-rate switching eliminates these problems and the screen first switches to the same frame-rate that the video is encoded in.
| 
