iPod Classic begins playing even it stays on hold

My iPod Classic 160 Gb does the strangest thing: when I play an audiobook and set it on pause and then on hold, after a while my iPod begins to play the same book further totally on its own. So when I after to or three hours take my iPod out to listen, I find it having been going much farther in the book. But that happens not always, very often, but not all the time. What can be the problem?