AN MD80 AUTOPLT MADE AN UNCOMMANDED TRANSITION FROM ALT HLD TO VERT SPD AT FL360.

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LEVEL AT FL360 IN CLR; SMOOTH CONDITIONS; AUTOPLT ENGAGED IN ALT HLD. NO WAVE; NO TURB. I JUST HAPPENED TO BE DOING AN INSTRUMENT SCAN WHEN I SAW ALT HLD SUDDENLY DISAPPEAR IN THE FMA; FOLLOWED BY VERT SPD. THROTTLES RETARDED AND ACFT BEGAN AN UNCOMMANDED DSCNT. 1400 FPM DOWN DISPLAYED IN PITCH WINDOW. I DIALED PITCH ANU TO COUNTER THE DSCNT; BUT THERE WAS NO RESPONSE FROM THE AUTOPLT AND THE WINDOW WAS FROZEN AT 1400 FPM DOWN; AND ACFT WAS IN AN UNCOMMANDED DSCNT. I CALLED THE CAPT'S ATTENTION TO THE PROB; DISCONNECTED AUTOPLT; AND HAND FLEW BACK TO FL360. AUTOTHROTTLES WERE ENGAGED IN MACH SEL AND THEY RESPONDED PROPERLY WITH A THRUST INCREASE TO MAINTAIN MACH AS I CORRECTED THE DSCNT AND RETURNED THE ACFT TO ALT. NEITHER OF US HAD OUR HANDS NEAR THE CTLS; AND THERE WAS NO WAY TO ACCOUNT FOR THE UNCOMMANDED DSCNT. THERE WERE NO ANNUNCIATIONS; WARNINGS; OR OTHER ABNORMAL ACFT/DFGS INDICATIONS. IT IS THE STRANGEST ANOMALY I HAVE EVER SEEN IN THIS AIRPLANE. I THINK I WAS ABLE TO ARREST THE DSCNT AT ABOUT FL357; AND THE ALT ALERT NEVER SOUNDED. ATC DID NOT QUESTION THE EXCURSION AND THERE WAS NO TFC CONFLICT. WE MONITORED AUTOPLT OPERATION VERY CLOSELY AND DISCUSSED THE POSSIBILITY OF USING ELECTRONIC INTERFERENCE PROCS; BUT THERE WAS NO REOCCURRENCE -- THE AUTOPLT FUNCTIONED NORMALLY THE REMAINDER OF THE FLT. I HAVE SEEN AUTOPLT DISCONNECTS IN WAVE; AND I HAVE SEEN REVERSIONS TO VERT SPD WHEN PERF DISENGAGED -- BUT NOTHING LIKE THIS. THE AUTOPLT WAS DRIVING THE AIRPLANE DOWN AT 1400 FPM FOR NO APPARENT REASON. IF I HADN'T HAVE BEEN STARING AT THE FMA WHEN IT HAPPENED WE MIGHT HAVE LOST A THOUSAND FEET OR MORE BEFORE WE CAUGHT THE PROB -- IT HAPPENED THAT QUICKLY. WE MADE A DISCREPANCY RPT. I'D BE INTERESTED TO FIND OUT IF THIS HAS HAPPENED BEFORE; AND WHAT DISCREPANCIES WERE UNCOVERED WITH THE ACFT; IF ANY.