Mark 1 - thanks for your comments.
Usually, the 'missed beat' happens when I'm on final, and just making a small alteration to keep on the correct glidescope, so the changes in power setting are both small and gentle. The problem also occurs when advancing to full throttle from low power settings. as happens when doing PFL's. At all other times, the engine runs very soothly. The engine was new 200hours ago.
T.O.