ACR MLG EXPERIENCES PITCHUP DUE TO AUTOPLT MALFUNCTION.

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I WAS THE CAPT OF AN ACR MLG ENRTE FROM THE ISLAND OF MAUI TO HONOLULU. THE REASON FOR THIS RPT WAS A DEV OF 1200-1700 FT FROM THE ASSIGNED ALT OF 16000 FT. THE CAUSE OF THE DEV WAS AN EMER SITUATION IN WHICH THE AUTOPLT WENT INTO A STRONG PITCH-UP RATHER THAN A MILD PITCH-DOWN WHEN IT SHOULD HAVE BEEN DOING ITS AUTOMATIC ALT CAPTURE MANEUVER. THE ACFT WAS CLBING AT 320 KIAS PER HONOLULU CTR'S INSTRUCTION. AT APPROX 14200 FT; A SLOW DECREASE IN CLB RATE WAS INITIATED SO THAT THE AUTOMATIC ALT CAPTURE WOULD OCCUR AT A LOWER CLB RATE AND YIELD A SMOOTH LEVEL OFF AT 16000 FT. AT APPROX 15600 FT (AS THE AUTOMATIC CAPTURE WAS BEING ANTICIPATED); THERE WAS A SHARP JOLT WHICH FELT LIKE AN EXAGGERATED VERSION OF THE JOLT WHICH OCCURS WHEN THE AUTOPLT IS DISENGAGED WHILE THE ELEVATOR SERVO IS APPLYING ELEVATOR INPUT. THE JOLT WAS ACCOMPANIED BY AN IMMEDIATE PITCHING UP OF THE ACFT AND THE RESULTANT INCREASE IN CLB RATE. I THOUGHT THAT WE HAD EXPERIENCED AN AUTOPLT DISCONNECT INSTEAD OF CAPTURE; AND PROCEEDED TO PUSH FORWARD ON THE YOKE TO LEVEL OFF. AS I APPLIED FORWARD PRESSURE; I WAS SURPRISED AT THE HIGH AMOUNT OF FORCE REQUIRED; AND INSTEAD OF TRIMMING IMMEDIATELY WITH THE THUMB SWITCH; I CHKED THE AUTOPLT ELEVATOR SERVO INDICATOR; AND NOTICED THAT IT WAS FULL SCALE UP AND THAT THE ELEVATOR OUT OF TRIM LIGHT WAS ON. AT THAT POINT I REALIZED THAT THE AUTOPLT HAD CAPTURED; AND NOT DISCONNECTED. I WAS UNABLE TO SAFELY DISCONNECT IT MANUALLY AT THAT POINT BECAUSE OF THE FLT ATTENDANTS AND SERVING CARTS IN THE AISLE. I THOUGHT THAT I HAD SUCCEEDED IN OVERPWRING THE AUTOPLT; AND HAD LEVELED AT 16200-16300; BUT THE FORCE INCREASED; AND I IMMEDIATELY INSTRUCTED THE FO TO 'CALL CTR AND TELL THEM WE'RE BUSTING ALTS'. I MADE A PA ANNOUNCEMENT TELLING THE PAX TO MAKE SURE THAT THEIR SEAT BELTS WERE SECURELY FASTENED BECAUSE WE WOULD PROBABLY BE GETTING A MUCH BIGGER JOLT THAN THE FIRST ONE VERY SHORTLY DUE TO AN AUTOPLT MALFUNCTION. THE FA WERE INSTRUCTED TO SECURE THE CABIN AND BE SEATED. I THEN WENT BACK TO A 2 HANDED PUSH ON THE YOKE AND TOLD THE FO TO MAKE ANOTHER REDUCTION. AS WE CLBED IN CAVU CONDITIONS WITH VERY LOW PWR; THE SPD BLED OFF RAPIDLY; AND THE AUTOPLT WAS SUCCESSFULLY DISCONNECTED AT APPROX 200 KIAS. THE ELEVATOR SERVO INDICATOR WAS STILL SHOWING FULL SCALE NOSE UP; BUT BECAUSE OF THE LOW SPD; THE JOLT WAS SMALLER THAN THE CAPTURE JOLT; AND THE PITCH-DOWN WAS SLIGHT. DUE TO THE 'SMOOTH' DISCONNECT; THERE WAS NO ACFT DAMAGE; AND NO INJURIES. THE FO AND I BOTH THOUGHT THAT WE HAD 'TOPPED OUT' BTWN 17200-17300 FT; BUT A PHONE CONVERSATION WITH HONOLULU CTR REVEALED THAT THEY RPTED 17700 FT.