A problem with a battery operated Koma clock?

I purchased a Koma battery operated (400-day type) clock in the 1970s while stationed in Germany. Recently the clock started showing a problem; specifically, the four-ball pendulum will slow down and stop only to gradually start up again after a period of time. What’s peculiar is that the clock keeps perfect time throughout these events. I have replaced the battery (only one is used with this particular clock) with no change in the variable pendulum activity. Can you advise what may be the cause and corrective measure for such fault?