A B757-200 LOSES AIRSPD SHORTLY AFTER REACHING CRUISE ALT AND HAS TO DSND 2000 FT TO REGAIN PROPER AIRSPD IN ZMP AIRSPACE.

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ATC INQUIRED IF WE WERE ABLE TO CLB TO FL410 (FOR TFC REASONS) ON A W TO E TRANSCONTINENTAL FLT. MAX SVC CEILING FOR OUR ACFT DUE TO WT RESTRS WAS FL400; AS A RESULT ATC REQUESTED US TO CRUISE AT FL390 TO ACCOMMODATE ANOTHER ACFT ON A SIMILAR RTE AT FL370. WE ACCEPTED THE CLRNC TO FL390. AN FMC CLB TO FL390 WAS INITIATED AND UPON REACHING FL390 WE PROGRAMMED A SLIGHTLY REDUCED MACH SPD TO MAXIMIZE THE BUFFET BOUNDARY AT THE NEW ALT. AT SOME POINT AFTER LEVELOFF AND STABILIZATION AT THE NEW SPD; AIRSPD BEGAN TO DISSIPATE AT AN EXTREMELY SLOW RATE THAT WENT UNDETECTED BY BOTH CREW MEMBERS. SOMETIME LATER WE FELT A VERY SLIGHT ACFT VIBRATION THAT WE INITIALLY ASSUMED TO BE CAUSED BY AN ENG BEING OUT OF SYNCHRONIZATION. WHEN WE REALIZED THE VIBRATION THAT WE HAD DETECTED WAS ACTUALLY THE ONSET OF LOW SPD BUFFET; WE IMMEDIATELY ADVANCED THE THROTTLES (WHICH WERE ALREADY AT A NEAR CRUISE PWR SETTING) AND INITIATED A SLOW DSCNT IN ORDER TO REGAIN REQUIRED AIRSPD. SIMULTANEOUSLY; I DIRECTED THE FO TO REQUEST A LOWER ALT FROM ATC. THE CTLR ADVISED THAT LOWER WAS NOT AVAILABLE DUE TO TFC AND REQUESTED THE NATURE OF OUR DIFFICULTY. WE ADVISED ATC THAT WE HAD AN AUTOPLT MALFUNCTION AND THAT WE WERE DSNDING IN ORDER TO AVOID AN ACFT UPSET AND WE WOULD ACCEPT A VECTOR OFF COURSE. WE WERE ABLE TO STABILIZE THE SPD OF THE ACFT AT FL382 AND TURNED TO THE CTLR'S OFF COURSE VECTOR HDG TO AVOID CONFLICT WITH THE TFC AT FL370. AFTER SEPARATION WAS ACHIEVED (INSURED) ATC CLRED US TO MAINTAIN FL370. WE DISCOVERED THAT THE AUTOTHROTTLES HAD SOMEHOW DISENGAGED DURING OUR CRUISE AT FL390. THE LAST PWR SETTING BEFORE THE DISENGAGEMENT OF THE AUTOTHROTTLES WAS JUST SLIGHTLY BELOW THE SETTING NECESSARY TO MAINTAIN PROPER AIRSPD. NO EICAS OR OTHER WARNINGS ACTIVATED (NORMALLY MASTER WARNING LIGHTS AND HORN) THAT WOULD ALERT US TO THE AUTOTHROTTLE DISCONNECT. AUTOTHROTTLES WERE RE-ENGAGED AND WORKED NORMALLY THROUGHOUT THE REMAINDER OF THE FLT. THIS FLT WAS CONDUCTED USING INTEGRATED FMC AUTOPLT OPS FROM 1000 FT AGL. THERE WAS NO CHANCE THAT EITHER PLT INADVERTENTLY INTERRUPTED THE PROGRAMMING SEQUENCE AND CAUSED THE AUTOTHROTTLES TO DISENGAGE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THEY NEVER GOT AN EICAS MESSAGE OR AN AURAL CAUTIONARY WARNING THAT THEY HEARD OR SAW WHEN THE AUTOTHROTTLES WERE DISENGAGED. HOWEVER; THE THROTTLE ANNUNCIATION WAS NOT OBSERVED. RPTR COULD NOT RECALL WHEN THAT ANNUNCIATION DISAPPEARED FROM THE ADI SCREEN. RPTR FEELS THE FLC WOULD HAVE HEARD THE WARNING SOUND OF DISCONNECT; BUT CANNOT EXPLAIN HOW THE THROTTLE ANNUNCIATION WOULD HAVE DISAPPEARED WITHOUT THEIR KNOWLEDGE. ACFT HAD BEEN AT CRUISE ALT ABOUT 15 MINS WITH DECAYING AIRSPD WHEN THE BUFFETING BEGAN.