I have a 172N with an S-TEC 60-2 AP with altitude pre-select and trim servo. The AP has been displaying some unusual behavior. For example, after engaging the HDG roll control, it won't fully disengage. Pressing the disengage button on the yoke, you still feel pressure from the servo. The AP doesn't respond to any changes in the HDG bug, but the servo remains in the position it was in when disengaged. Turning the AP off at the on/off/test switch causes no change in this. Pulling the AP circuit breaker results in no change either. If the pitch (elevator) servo was engaged, it, too, will remain in the position it was in when disengaged. The servos don't release until the aircraft is back on the ground. I've labeled the unit INOP and pulled the breaker for safety reasons. Any suggestions anyone has will be appreciated.