On August 15, 2012, about 0835 central daylight time, a Cessna 560XL, N108EK, diverted to Des Moines International Airport (DSM), Des Moines, Iowa, following a loss of control during cruise at Flight Level (FL) 400. The two pilots and six passengers were not injured. The airplane sustained minor damage. The aircraft was registered to New Heights Aviation LLC and operated by Elliott Aviation under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed for the flight, which was operated on an instrument flight rules flight plan. The flight originated from Flying Cloud Airport (FCM), Eden Prairie, Minnesota, at 0748, with an intended destination of Arlington Municipal Airport (GKY), Arlington, Texas.The flight crew reported that after about 10 minutes of cruise flight at 40,000 feet pressure altitude, they observed an autopilot pitch trim mis-trim (AP Pitch Mis-trim) annunciation and associated master caution. As they began to reference the applicable checklist, the autopilot automatically disengaged and the airplane suddenly pitched down to about 20 degrees. The airspeed increased to about 30 knots over redline. The flight crew recovered and leveled the airplane at 33,000 feet pressure altitude. They subsequently elected to divert to DSM and landed without further incident. The autopilot was not used for the duration of the flight. However, the electric pitch trim was used passing 10,000 feet mean sea level and it functioned normally. The flight crew noted that an "abnormally high amount of water" was observed to be draining from the tailcone during the postflight inspection. The cockpit voice recorder (CVR) was downloaded and a summary of the recording was prepared. The CVR summary revealed that as the airplane was leveling at flight level (FL) 400, the co-pilot noted "it's not keeping up with it." He commented about referencing the checklist and added "R-five." About 30 seconds later, a high-pitched tone was recorded simultaneously with the sound of a "thunk" and rustling sounds. Two additional high pitched tones were recorded. These were followed by a beeping tone consistent with the overspeed warning, which lasted for 40 seconds. The flight crew discussed deploying the speed brake and declared an emergency. The flight crew regained control of the airplane and diverted to DSM. During the descent, the crew discussed that they would not use the autopilot for the duration of the flight. Based on further discussions recorded by the CVR, the pilots executed a full flap approach. They subsequently landed at 0844 without further incident. The required pilot actions related to an AP Pitch Mis-trim annunciation were contained on page R-5 of the Pilots' Abbreviated Checklist – Emergency/Abnormal Procedures. The procedure called for: (1) AP Trim Disc Button – Press and Release (if elevator trim not in motion); Pitch Trim – Adjust (as required); and Autopilot – Engage (as desired). A postincident examination of the autopilot, pitch trim systems and two-position horizontal stabilizer system did not reveal any anomalies. The left and right elevator electric trim actuators and the autopilot elevator drive servo were subsequently replaced. The airplane was returned to service without any further difficulties.