ACR FLC CLBED MLG ABOVE OPTIMUM CEILING. COULD NOT MAINTAIN ALT.

Narrative

ENRTE FROM SMF TO LAX AN MLG WAS CLRED TO CLB TO FL370. AS FL350 CAME AND WENT; IT BECAME EVIDENT THAT THE ACFT WAS NEARING ITS CEILING SO BOTH THE CAPT AND I LOOKED AT THE PERFORMANCE NUMBERS WHICH STATED THE OPTIMUM CEILING WAS FL360 AND MAX CEILING FL370. SO WE CONTINUED CLB TO FL370 WITHOUT INCIDENT. AFTER LEVEL OFF; THE AUTOPLT WAS PLACED ON AND SHORTLY THEREAFTER THE CLRNC TO A NEW HDG 30 DEGS TO R WAS GIVEN. WITH A BANK ANGLE OF 15 DEGS SELECTED THE AUTOPLT BEGAN THE TURN. BOTH PLTS OBSERVED THIS THEN SET ABOUT OUR OTHER DUTIES. A FEW SECONDS LATER BOTH PLTS LOOKED UP TO OBSERVE THE ALT SLOWLY DSNDING THROUGH FL36.8. AT THAT POINT I DISCONNECTED AUTOPLT AND TRIED TO REGAIN THE ALT TO NO AVAIL. THE ACFT CONTINUED TO SINK ON THE EDGE OF A STALL BUFFET; UNABLE TO HOLD ALT. AFTER FIRST BEING DENIED A LOWER ALT; THE CAPT AGAIN STATED THAT WE WERE UNABLE TO MAINTAIN FL370 AND A CLRNC TO FL330 WAS RECEIVED. WE BELIEVE THAT EITHER THE ACFT WAS HEAVIER THAN INDICATED (I.E.; HEAVIER CARGO) AND OUR NUMBERS WERE INCORRECT. NEXT TIME WE WILL BOTH BE MORE AWARE OF THE ACFT'S WARNING SIGNS.