B767-300 THROTTLES RETARDED WITHOUT THE FLT CREW'S AWARENESS. THE AIRSPEED DECREASED; STICK SHAKER ACTIVATED; AND THE ACFT LOST APPROX 1300 FT WHILE AIRSPEED WAS REGAINED.

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SHORTLY AFTER THE TOP OF CLB AT FL300; THE STICK SHAKER ACTIVATED. AT FIRST; I THOUGHT THAT IT MIGHT BE AN ERRONEOUS ACTIVATION; BECAUSE THE ACFT ATTITUDE LOOKED NORMAL; WE WERE IN LEVEL FLT; AND I HAD NOT NOTICED ANY CHANGE IN ACFT WIND NOISE; PITCH ANGLE; OR TRIM. I NOTED THAT MY AIRSPD WAS APPROX 220 KIAS. I CALLED OUT TO THE FO; WHO WAS PF; THAT IT MIGHT BE A FALSE STICK SHAKER. I THEN NOTED HIS AIRSPD; WHICH WAS ALSO VERY LOW. I CALLED OUT LOWER THE NOSE; BUT BY THIS TIME HE HAD ALREADY ADVANCED THE THROTTLES; DISCONNECTED THE AUTOPLT AND BEGUN A DSCNT. WE ALSO HAD INOP EEC'S TO CONSIDER. I INFORMED BRASILIA CTR OF THE SITUATION; AND THEY CLRED US INTO AIRSPACE FROM 280 TO 300. WE REGAINED AIRSPD AND RETURNED TO NORMAL CRUISE. OUR LOWEST ALT WAS APPROX 28700 FT. AS WE WERE DISCUSSING WHAT MIGHT HAVE HAPPENED; THE AUTOTHROTTLES STARTED TO PULL THE THROTTLES BACK FOR NO REASON. AT THIS POINT; WE DISCONNECTED THEM. THIS ACFT HAD A RECENT HISTORY OF AUTOTHROTTLE MALFUNCTIONS; MOST OF WHICH DESCRIBED UNCOMMANDED DISCONNECT. HOWEVER; THERE WAS ONE DESCRIBING AN AUTOTHROTTLE MALFUNCTION LIKE WE HAD; BUT I WAS NOT AWARE OF IT BECAUSE IT WAS FURTHER BACK THAN I HAD REVIEWED. I AM GUESSING THAT THE AUTOTHROTTLES HAD RETARDED FROM CLB TO A BELOW CRUISE PWR SETTING; AND IN THE DARK WE DID NOT DETECT THE THROTTLE POS. AT ABOUT THIS TIME; I WAS PREPARING MY CHARTS FOR POS RPTS; AND THE FO WAS EDITING THE FMS. ALTHOUGH I ALWAYS ATTEMPT TO MAKE SURE ONLY 1 PLT IS HEADS DOWN AT A TIME; I CANNOT SAY FOR SURE WHAT HAPPENED IN THIS CASE. IN RETROSPECT; I PROBABLY SHOULD NOT HAVE ACCEPTED THIS TRIP; WHICH STARTED OUT IN ZZZ1 AND REQUIRED ME TO DEADHEAD ON VERY SHORT NOTICE TO ZZZ2 AND FLY THE ALL-NIGHTER TO SBGL. I WAS AWAKE FOR ALMOST 30 HRS; FOLLOWED BY A 34 HR LAYOVER AND ANOTHER ALL-NIGHTER; DURING WHICH THIS EVENT HAPPENED. OBVIOUSLY; I WAS NOT PERFORMING AT MY BEST.