FLC OF AN MD83 DISCONNECTED THE AUTOPLT WHEN IT INADVERTENTLY DISENGAGED FROM ALT HOLD AND CLBED THE ACFT FROM ASSIGNED. THEY THEN RETURNED THE ACFT TO ASSIGNED ALT AND ADVISED ATC AS TO WHAT HAPPENED.

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I WAS FO AND PF ON THIS FLT AND HAD BEEN CLRED TO FL280. THE AUTOPLT WAS ENGAGED AND WAS CTLING VERT FLT DURING THE CLB AND THROUGHOUT THE SUBSEQUENT LEVELOFF AT THE ASSIGNED CRUISE ALT. BOTH MYSELF AND THE CAPT VERIFIED AND VERBALIZED 'ALT CAPTURE;' AND 'ALT HOLD' ON THE FMA. A SHORT WHILE LATER THE 'ALT' WARNING SOUNDED IN THE COCKPIT. I WAS JOINED IMMEDIATELY ON THE CTLS BY THE CAPT WHOM ADVISED ATC OF THE AUTOPLT MALFUNCTION. WE SIMULTANEOUSLY DISCONNECTED THE AUTOPLT AND HAND FLEW THE AIRPLANE IN THE DSCNT BACK TO THE ASSIGNED ALT. THE 'ALT' ALERT SOUNDS AT APPROX 300 FT DEV FROM THE ASSIGNED ALT. BY THE TIME WE WERE ABLE TO DISCONNECT THE AUTOPLT AND ARREST THE CLB MANUALLY; HOWEVER; THE AIRPLANE HAD CLBED AN ADDITIONAL 100-200 FT; RESULTING IN A TOTAL ALTDEV OF 400-500 FT. IT IS POSSIBLE THAT I INADVERTENTLY DISCONNECTED THE PITCH FUNCTION OF THE AUTOPLT BY TOUCHING THE MANUAL PITCH TRIM BUTTON ON THE CTL WHEEL; ALTHOUGH I HAVE NO RECOLLECTION (OR NEED) OF DOING SO. ANOTHER POSSIBILITY IS THAT THE AIRPLANE WAS OUT PITCH TRIM LIMITS ABOUT THE LATERAL AXIS WHICH COULD HAVE CAUSED THE ALT HOLD FUNCTION OF THE AUTOPLT TO DISENGAGE. THE EVENT REMAINS A MYSTERY TO BOTH THE CAPT AND MYSELF.