Then probably one of the 4 autopilot actuators (2 aileron and 2 elevator) pressure switches is faulty.
This pressure switch senses hydr. pressure after the actuator is engaged by the autopilot. (feedback signal) However, the time from engaging the a/p and actuation of the pressure switch takes approx 3.5 seconds.
And when this switch fails, what happened in the past, then the a/p disconnects again.
So before replacing any black boxes or doing extensive a/p tests, first check this switch which you can find on each a/p actuator.
