AN ACR LTT COULD NOT MAINTAIN A NEWLY ASSIGNED HIGHER ALT.

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DUE TO TURB AT FL310 WE REQUESTED A CLB TO FL350. OUR PERFORMANCE CHARTS INDICATED WE WOULD BE ABLE TO MAINTAIN 35000 FT AT OUR EXISTING WT. AT 33500 FT CTR REQUESTED US TO EXPEDITE TO FL350. WE DID THIS RESULTING IN AN AIRSPD DECREASE TO APPROX 230 KIAS. UPON REACHING FL350 WE WERE UNABLE TO ACCELERATE AND IN FACT BEGAN TO LOSE AIRSPD AS WELL AS EXPERIENCE LIGHT BUFFETING. WE BEGAN AN IMMEDIATE DSCNT CALLING CTR 3 TIMES BEFORE SUCCESSFULLY GETTING THROUGH AND RECEIVING A CLRNC BACK TO FL310. BY THIS TIME WE WERE AT FL340. CONTRIBUTING CAUSES: OUR 150 BAGS IN CARGO APPEARED HEAVIER THAN THE 24 LBS/PER USED FOR WT AND BAL CALCULATIONS. COCKPIT TEMP INFO IS ONLY IN 'RAT' FORMAT. PERFORMANCE CHARTS USE STATIC TEMPS. CONSEQUENTLY YOU MUST CONVERT THE 'RAT' TO STATIC OUTSIDE AIR TEMP TO UTILIZE THE CHARTS. OUR CONDITIONS WERE 22 DEGS CELSIUS ABOVE STANDARD. SUPPLEMENTAL INFO FROM ACN 252969: WITH 1000 FT TO GO; ATC ASKED US TO PICK UP THE RATE. I INCREASED THE RATE FROM 300 FPM TO 500 FPM AND LOST 10 KTS DOING SO. AT LEVEL OFF; I LEFT THE THRUST AT MAX CLB AND EXPECTED THE ACFT TO ACCELERATE. TO MY SURPRISE IT BEGAN TO DECELERATE AND LIGHT PRE-STALL BUFFET WAS FELT. AT JUST 10 LBS MORE PER PAX AND BAG; THE EXTRA 2700 LBS MAY HAVE MADE IT IMPOSSIBLE TO SUSTAIN FL350!