Those of you who are experiencing jerky playback in the UHD version: Try remuxing the streams into a new MKV container - somebody on another forum did this when adding/subtracting some audio streams and the problem went away. It was originally muxed with the latest version MKVToolnix, but perhaps there is a glitch in the container. Try it and let us know if that fixes the problem for you too.
I tried that when I first noticed the problem. Though I probably also had the latest mkvtoolnix, I did mux it with only a single audio track. I also tried m2ts. Jerky playback with everything. As I’ve said once or thrice, MadVR in MPC solved the problem completely.
Thanks, I’m kind of relieved that’s not the problem…