As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FOR ABOUT THREE YEARS MY AUTOPILOT WILL ABRUPTLY CLIMB AND DESCEND UP TO 300 FEET IN ALTITUDE HOLD AND WILL CLIMB AND DESCEND IN A SET VERTICAL RATE CLIMB OR DESCENT. I HAVE FOUND THAT IF I KEEP AN ALLEN HEX KEY IN THE AIRPLANE SO THAT IF THE AUTOPILOT STARTS TO CLIMB AND DESCEND IN ALTITUDE HOLD, IF I TIGHTENED UP THE ALLEN HEX SCREW THE AUTOPILOT WILL STOP THE DEVIATION FROM ALTITUDE FOR A WHILE. THIS AUTOPILOT FLYING IS UNCOMFORTABLE AND DANGEROUS IFR.