A mistrim condition while the autopilot was engaged for reasons that could not be determined based on available evidence, and the pilot’s subsequent failure to confirm the command bar status before disengaging the autopilot. Contributing to the accident were the pilot’s misinterpretation of the event based on a previous unrelated experience, a jammed trim switch, and autopilot disconnect switch which broke off in flight, and the stress induced by the physical exertion required to maintain control of the airplane.