ALTDEV ALT EXCURSION WHEN AUTOPLT MALFUNCTIONS.

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WE WERE ON FLT MCO-BOS. ORIGINALLY AT FL410; WE HAD BEEN DIRECTED TO DSND TO FL230. WE DSNDED AND WERE LEVEL AT FL230. CAPT WAS FLYING WITH FMC SET TO FL230; LEFT AUTOPLT ENGAGED. AFTER ABOUT 5 MINS OF LEVEL FLT; WITHOUT WARNING AND UNCOMMANDED; THE ENGS ACCELERATED TO CLB EPR AND THE ACFT STARTED A CLB AT GREATER THAN 2000 FPM. THE ONLY WARNING WAS THE ALT ALERTER AS WE CLBED GREATER THAN 300 FT OVER FL230. THE CAPT DISCONNECTED THE AUTOPLT AND AUTOTHROTTLES IMMEDIATELY; PUSHED THE NOSE OVER TO RETURN TO ALT AND EASED BACK TO FL230. WE PROBABLY EXCEEDED OUR ASSIGNED ALT BY 600-700 FT (FL23.7). I HAVE NO IDEA WHY THE AUTOPLT MALFUNCTIONED IN THIS MANNER WITH NO EICAS WARNINGS OR MESSAGES. ONCE BACK AT FL230; WE RE-ENGAGED THE SAME AUTOPLT FOR THE REMAINDER OF THE FLT. THERE WERE NO FURTHER PROBLEMS. WE WROTE UP THE MAINT DISCREPANCY UPON ARR AT BOS. IN THIS INSTANCE; THE HUMAN ELEMENT INTERVENED AND CORRECTED WHAT COULD HAVE BECOME A VERY SERIOUS MECHANICAL EXCURSION. ATC QUERIED OUR ALT AS WE DSNDED THROUGH 23.2 BACK TO FL230. I DO NOT BELIEVE THERE WAS A CONFLICT AS WE HAD JUST DSNDED FROM FL240 TO FL230 MINS PRIOR TO THIS INCIDENT.