Autopilot wing rock problem

During the last couple of flights, with the autopilot engaged, I have experienced uncontrolled, rythemic wing rocking that stops when the A/P is disengaged. It has occurred an hour or two into the flight and seems to have been triggered both times by some turbulance. It is the original Piper AutoControl III A/P on a 1976 235. Once the problem starts, it occurs no matter what mode the autopilot is in (e.g, heading bug control, roll selector, etc.). When it is re-engaged later in smooth air, it seems to work fine. When it has occurred, I cannot get it to stop using the yoke, but only by turning it off. Any experience with this and insight into the cause and solution?
Thanks
Tom