Not sure if I'm using the program incorrectly or if this is a bug. Created mkv with h264 codec. Jerky playback with the popcorn hour nmt. After demuxing it, instead of finding h264 file and sound file, found an avi file and sound file. The avi file contained the .h264 extension. Looks like a container within a container. Other forums suggest h264 not suited to avi container. So using yamb, I xtracted the h264 and then muxed it back with the sound file. Playback much smoother. I've noticed other users with similar nmt having similar problems.