Autohelm 4000 Intermittently Dives Port or Starboard

[senormechanico]

I have a 1995 vintage Autohelm 4000 which occasionally makes a rather extreme course correction (20-30 degrees) and gradually returns to its original heading. Usually I disconnect it and reset the course before it resets.

I have Raytheon wind direction/speed/depth/VMG/multi instruments and a ST600R remote on the Seatalk line as well as a fluxgate compass hooked to the autopilot via the NMEA connections.

With all electronics except the autopilot turned off, it still does this.
It happens intermittently every ten minutes or so with no apparent pattern or reason such as big waves etc.

Any ideas appreciated.

Steve B.