B757 CREW UNABLE TO MAINTAIN ALT BECAUSE OF ICING; BEGINS DSCNT BEFORE COORD WITH ATC CAN BE COMPLETED.

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WE HAD JUST CLBED TO FL410 IN THE AREA OF ABQ. FMS SAID WE WERE OK TO FL414. WE WERE LEVEL AND STABILIZED AT FL410 AND .80 MACH. WE THEN ENTERED SOME CLOUDS AND BEGAN TO EXPERIENCE SOME LIGHT CHOP. THE AIRSPD WAS FLUCTUATING A LITTLE BUT NOT BADLY. THEN I NOTICED THAT THE AIRSPD BEGAN TO DROP OFF BUT THE AUTOTHROTTLE WAS NOT ADVANCING THE PWR. I THEN PUSHED THE THROTTLES UP MANUALLY AND THE AUTOTHROTTLE RETARDED THEM. I COULD NOT UNDERSTAND WHY; SO I SELECTED ALT HOLD AND CONTINUED ON THRUST MGMNT COMPUTER AND THEN PUSHED THROTTLES UP AGAIN. AIRSPD WAS ABOUT .745 MACH BY NOW; SO I TOLD FO TO REQUEST LOWER AND I BEGAN A SLOW DSCNT. ATC SAID STAND BY OF COURSE. HE TOLD THEM WE WERE HAVING TROUBLE MAINTAINING AIRSPD AND NEEDED LOWER. I HAD DSNDED TO ABOUT FL407 BY THE TIME WE WERE CLRED TO FL370. THE CTLR DID NOT MENTION THE ALTDEV (NO TFC CONFLICT). AFTER DISCONNECTING THE AUTOTHROTTLE; WE TURNED WING AND ENG ANTI-ICE ON. THE SAT WAS -56 DEGS C; SO ANTI-ICE WAS NOT ON. I THEN RECONNECTED AUTOTHROTTLE AFTER A SHORT WHILE AND AUTOTHROTTLE WORKED FINE AFTER THAT. AFTER CTL WAS RE-ESTABLISHED; WE DID NOTICE THAT THERE WAS A LITTLE ICE ON THE WIPER BLADES. I DO NOT KNOW IF ICING WAS OUR PROB. OUR POM STATES THAT ANTI-ICE SHOULD BE USED EXCEPT WHEN SAT IS BELOW -40 DEGS C. IF ICING DID CAUSE THIS THROTTLE PROB AND LOSS OF AIRSPD; THEN OUR POM SHOULD BE CHANGED. FROM NOW ON; I WILL ALWAYS USE ANTI-ICE BELOW 10 DEGS C.