AN ACR B-747 GOT BEHIND THE PWR CURVE AND NEARLY STALLED.

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WE LEVELED AT FL340 75 MI E OF CLUTS INTXN; SPD AT ABOUT MACH .79 AND EXPECTED ACFT TO ACCELERATE TO MACH .84. WE WERE AT MAX CLB PWR BUT ACFT SLOWED; STABILIZING AT MACH .72 FOR ABOUT 10 MINS. NEARING CHEAK INTXN AND REALIZING WE WERE NOT GOING TO ACCELERATE; WE REQUESTED FL320 FROM ATC THROUGH HNL RADIO (HF). IT WAS DENIED BECAUSE OF TFC. WE BEGAN TO EXPERIENCE SOME WAVE ACTIVITY AND TURB. OUR AIRSPD STARTED TO DECREASE. THE FO WAS FLYING ON AUTOPLT AND WAS TRYING TO TRADE SOME ALT FOR AIRSPD TO NO AVAIL. AT THIS TIME WE WERE ABOUT 650000 LBS; WHICH IS 9000 LBS BELOW MAX CRUISE WT FOR STANDARD TEMP PLUS 10 DEGS AT 340. WE COULD NOT UNDERSTAND WHY WE COULD NOT MAINTAIN SPD AND ALT. THE FO DISENGAGED THE AUTOPLT AND THE CAPT TOLD HIM TO DSND TO FL335 AND TRY TO GET AIRSPD BACK. AT FL335 THE ACFT PITCHED UP RAPIDLY TO FL340 WITH FURTHER LOSS OF AIRSPD TO MACH .67. WE WENT THROUGH ANOTHER CYCLE FROM FL340 TO FL335 TO FL340 WITH MORE AIRSPD LOSS AND THE CAPT TOOK CTL OF THE ACFT. WE WERE STILL IN WAVE ACTION AND LIGHT TURB AND PITCH INSTABILITY. THE CAPT HAD NO BETTER LUCK CTLING THE ACFT THAN THE FO HAD. AFTER ANOTHER CYCLE FROM FL340 TO FL335; WE WERE STILL DSNDING THROUGH FL335 AND SLOWING BELOW 250 KIAS. THE CAPT OBSERVED ON TCASII TFC WAS APPROX 10 MI BEHIND US AND TO OUR L AND 1500 FT BELOW US WHEN WE GOT THE FIRST STICKSHAKER. THE CAPT TOLD THE FO TO TELL HNL THAT WE WERE TURNING R; OFF COURSE; AND DSNDING TO FL300 AND DECLARING AN EMER. IT WAS HARD TO TELL IF WE WERE IN TURB OR IN A STALL BUFFET. WE DSNDED RAPIDLY WITH MAX CONTINUOUS PWR SET IN ORDER TO GET BELOW TFC AND TRY TO GET THE SPD BACK. WE LEVELED AT FL300 AND MACH .84 AT 10 MI R OF COURSE. WE COORDINATED WITH HNL RADIO FOR FL300 AND A RETURN TO ON COURSE. AT FL300 WE COULD HOLD MACH .84 AT NORMAL CRUISE PWR AND WE ADVISED HNL RADIO THE EMER WAS OVER. CONCLUSION: I THINK THERE MAY HAVE BEEN AN ERROR ON THE WT MANIFEST; MAKING OUR GROSS WT SOMEWHAT MORE THAN STATED; ALTHOUGH THIS DOES NOT APPEAR TO BE THE CASE. ALSO I FEEL NOW THAT IF THERE IS WAVE ACTION AND TURB; THE CREW SHOULD PICK A CRUISE ALT NOT BASED ON MAX CRUISE WT BUT ON OPTIMUM CRUISE WT.