WDB CARGO ACFT MAINTAINING FL350 WELL WITHIN WT LIMITS. HOLDING AT HPW. HAD COMPLETED ONE CIRCUIT IN THE HOLD; WHEN AIRSPD BEGAN TO SLOW AND WAS UNABLE TO REGAIN. REQUESTED LOWER ALT BUT CTR COULD NOT GIVE UNLESS EMER DECLARED. DECLARED EMER AND DSNDED TO FL330 WHERE AIRSPD WAS REGAINED.

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WE WERE FLYING FROM MEM TO EWR AT FL330. CTR ADVISED US OF HOLDING AHEAD AND ASKED IF WE COULD CLB TO FL350. OUR WT WAS 370000 LBS. MAX AT FL350 WAS 400000 LBS. WE CLBED TO FL350 AND ENTERED HOLDING AT HOPEWELL VOR. SO ADVISED US THAT V MINIMUM SLATS RETRACT SPD WAS 230 KIAS. WE DECIDED TO HOLD AT 240 KIAS. FIRST TURN IN HOLDING WAS NORMAL. ON SECOND TURN; AIRSPD BEGAN TO DECREASE RAPIDLY. I LIMITED BANK TO 15 DEGS AND HIT TKOF PWR ON THRUST COMPUTER. WITH WINGS LEVEL; AIRSPD STABILIZED AT 200 KIAS. I ASKED CTR FOR LOWER. HE SAID NO; THERE WERE 5 ACFT HOLDING BELOW US. I SAID WE COULD NOT HOLD FL350 AND ASKED IF FL340 WAS AVAILABLE. HE SAID NO; HE NEEDED 2000 FT SEPARATION. I AGAIN SAID WE WERE UNABLE TO HOLD OUR ALT AND NEEDED LOWER. CTR SAID UNABLE; UNLESS 'YOU DECLARE AN EMER.' I DECLARED AN EMER AND ADVISED CTR THAT WE WERE HDG 360 DEGS AND LEAVING FL350. ONCE WE BEGAN THE DSCNT; AIRSPD INCREASED RAPIDLY AND WE LEVELED AT FL330 AND ADVISED CTR OF END OF EMER. THE ACFT AT FL350 WAS IN THE NIBBLE OF BUFFET. AUTO SLATS DID NOT EXTEND; ACFT DID NOT STALL; BUT WE WERE ON THE BACK SIDE OF THE PWR CURVE AND COULD NOT REGAIN AIRSPD WITHOUT DSNDING. I SHOULD HAVE TAKEN COMMAND OF ACFT; DISCONNECTED AUTOPLT; LEVELED WINGS AND USED MAX PWR. INSTEAD; I WATCHED FO ATTEMPT TO CTL ACFT ON AUTOPLT.