AN MD80 IN CRUISE AT FL350 EXPERIENCED A VERY SLOW REDUCTION IN PWR FROM PT 77 MACH TO PT 70 MACH WHILE IN FLT MGMNT PERFORMANCE MODE. CAUSE UNKNOWN.

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APPROX 2 HRS INTO FLT; THROTTLES RETARDED TOWARD FLT IDLE WITHOUT INPUT FROM CREW. THIS WAS A VERY SLOW REDUCTION AND WENT UNNOTICED UNTIL THE TRIM C TONE BEGAN SOUNDING AND THE FO SAW THE CDU MESSAGE LIGHT FLASH. THE FO STATED THE PERFORMANCE MANAGEMENT SYSTEM (PMS) FLASHED A MESSAGE BUT THE MESSAGE WAS GONE BEFORE IT COULD BE READ. PERFORMANCE CRUISE MODE WAS BEING USED AT THE TIME WITH .77 MACH SELECTED. THE SPD SELECT WINDOW ON THE DIGITAL FLT GUIDANCE COMPUTER (DFGC) PANEL WAS ALSO SELECTED TO .77 MACH AS A BACKUP TO THE PERFORMANCE. AFTER THE THROTTLES RETARDED ON THEIR OWN; I LOOKED DOWN TO SEE WHAT THE PERFORMANCE WAS READING. IT WAS STILL IN THE CRUISE MODE WITH .77 MACH DISPLAYED. I LOOKED AT THE DFGC PANEL AND THE MACH WINDOW SHOWED .72 MACH. THE FMA STILL INDICATED PERFORMANCE CRUISE. I DISCONNECTED THE AUTOTHROTTLE AND PUSHED THE PWR UP MANUALLY TO APPROX 2.00 EPR OR MAX CRUISE. BY THE TIME THE PWR HAD BEEN PUSHED UP; THE ACFT HAD SLOWED TO .70 MACH. IT ACCELERATED STEADILY WITH CRUISE PWR SELECTED BACK TO .77 MACH. PERFORMANCE USE WAS DISCONTINUED. WE PUT ENG AND AIRFOIL HEAT ON AS A PRECAUTION -- THOUGH WE WERE IN THE CLR WITH NO PRECIP VISIBLE. ALL ENG INDICATIONS WERE NORMAL AND THERE WAS NO THROTTLE SPLIT. OUTSIDE AIR TEMP WAS APPROX -50 DEGS. THE FO WAS VERY ALERT AND BROUGHT THE SIT TO MY ATTN BY STATING 'WHAT'S THIS THING DOING?' WE STILL DON'T KNOW WHAT IT DID AND MAINT RAN TESTS ON THE PMS WITHOUT ANY FAULTS FOUND.