B757 FLT PERFORMANCE AT FLT PLANNED ALT INDICATE POSSIBLE ERROR IN CLOSEOUT WT. PERFORMANCE NORMAL AFTER 4000 FT DSCNT.

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ACFT CLBED SOMEWHAT SLOWLY TO FL370. AFTER LEVELOFF IN VNAV; FO REMARKED 'WATCH THE AIRSPD.' I LOOKED UP TO SEE AIRSPD DECELERATING THROUGH 240 KTS (.76 MACH) WITH PWR BACK AT 1.4 EPR. I PUSHED THE PWR UP AND CHKED EVERYTHING. AUTOTHROTTLES WERE ON; VNAV PATH WAS ENGAGED; AUTOPLT WAS ENGAGED; AND THE ACFT WAS WINGS LEVEL (NO TURN). ACFT WEIGHED 215000 LBS. ACFT SLOWLY ACCELERATED TO ABOUT .79 MACH AND WOULD NOT GO ANY FASTER. CRUISE MACH WAS .80 MACH. AUTOTHROTTLES WERE AT MAX CRUISE SETTING (APPROX 1.73 EPR). AFTER ABOUT 30 SECONDS THE AUTOTHROTTLES CAME BACK AGAIN TO ABOUT 1.5 EPR AND THE CYCLE LOOKED LIKE IT WAS GOING TO REPEAT ITSELF. AFTER I PUSHED THE THROTTLES BACK UP; WE NOTICED THAT THE DECK ANGLE AT .79 MACH WAS JUST OVER 5 DEGS NOSE UP. I ASKED THE FO TO REQUEST FL330 AND WE IMMEDIATELY DSNDED. AFTER REACHING FL330 NO FURTHER PROBS NOTED OF ANY KIND IN ANY AUTOFLT MODE. WE SPENT SOME TIME CHKING EVERYTHING: CLOSEOUT WT VERSUS FMS WT; PERFORMANCE CRUISE CHARTS VERSUS WT AND TEMP; AND WINDS ALOFT AND TEMP FOR ANY SIGNS OF MOUNTAIN WAVE. ALL THE DATA INDICATED THAT THE ACFT SHOULD HAVE FLOWN FINE AT FL370 EXCEPT THE DECK ANGLE. AUTOTHROTTLES AND AUTOPLTS SEEMED TO BE FINE AT FL330. I CONTACTED DISPATCH THROUGH ACARS AND HE SET UP CONFERENCE CALL WITH TECHNICIAN AND WE DISCUSSED THE WHOLE SIT ON THE RADIO. I THEN REQUESTED A LOAD AUDIT BE ACCOMPLISHED IN MIA TO VERIFY THE ACFT WT. I ALSO ADDED ABOUT 5 KTS TO MY APCH SPD IN MIA IN CASE WE WERE HVY (VREF +10 KTS). APCH FELT NORMAL.