Posted for Walter Atkinson <walter@advancedpilot.com>:

 I solved a similar problem in my Bonanza with the same autopilot by running
the servo in each direction for about 30 seconds.  These servos sit in one
spot and move VERY little back and forth.  They can carbon up slightly and
result in a higher signal to move them.  These work on very low voltages and
can become resistent.  This is seen as porpoising and sudden inputs as the
voltage gets high enough to start the carboned-up servo.  This has solved this

issue for me in three different aircraft.
 
 Disconnect the servo from the control cable.  Run it in one direction for
about 30 seconds.  Reverse the polarity and run it in the other direction for
about the same length of time.  Hook it back up.  This has solved my problem
which sounds like it could be the same as yours.
 
 Walter
 
 > The porpoising has been sporadic since first flight