No warnings in the log... That means that file is OK. Only the reason left - is very demanding video file. Really, it is strange that Stats window didn't show any rendering lag when you see it by your own eyes. You could try: to reduce bitrate of the source file, its resolution, try to use 4:2:0 subsampling (if something else specified) for the file, use fast hard disk drive to store the source file - all this makes it less demanding on playback.