DECLARED EMER RESULT OF MOUNTAIN WAVE.

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CRUISING AT FL410; MACH .80 ON J84 60 W DEN. PREVIOUS CONDITIONS OFF AND ON LIGHT TURB WITH LIGHT TO MODERATE TURB RPTED DOWN THROUGH FL290. WINDS AT 410 ABOUT 270/60. OBSERVED ACSL'S TO S AND WAS AWARE OF TURB PIREP SW OF DEN. ENCOUNTERED SEVERE WAVE ACTION RESULTING IN AIRSPD FLUCTUATIONS TO IN EXCESS OF .86 MACH (ACCOMPANIED BY OVERSPD WARNING) FOLLOWED BY RAPID DECELERATION TO 200 KIAS. ONLY LIGHT TURB ENCOUNTERED; BEGAN DSCNT WITHOUT CLRNC FROM ATC AS STALL WAS BELIEVED IMMINENT. ATC WAS NOTIFIED OF DSCNT AND CLRED FLT TO FL370 AS IT WAS TRANSITIONING FL400. TCASII WAS AVAILABLE AND IN USE AND HELPED ESTABLISH CLR TRACK FOR DSCNT (NO CONFLICTS WERE INVOLVED). WHEN ACFT ACCELERATED; IDLE PWR AND FULL SPD BRAKES WOULD NOT SLOW ACFT OUT OF OVERSPD REGIME AT ASSIGNED ALT. AS ACFT SUBSEQUENTLY SLOWED; MAX CRUISE PWR WOULD NOT ACCELERATE THE ACFT AND IT WAS FEARED THAT DOWNSTREAM TURB MIGHT CAUSE A STALL. A DSCNT WAS INITIATED TO REGAIN SAFE FLYING SPD. FO HAD RPTED FL410 WINDS AS 270/006. AS PLANE ENTERED ACCELERATION PHASE OF ENCOUNTER; FLT CONTINUED TO DEST CRUISING AT FL370 TILL WELL E OF DEN WHERE CLB TO FL410 WAS EXECUTED. WINDS AT FL410 E OF DEN WERE IN THE 270/60 RANGE AGAIN.