FLT CREW OF CARJ ACCEPT EXPEDITED CLRNC FROM FL350 TO FL390 IN DRVSM AIRSPACE. DUE TO MOUNTAIN WAVE AND TURB; THEY STOP CLBING BTWN FL380 AND FL390. FORCED CLB TO AVOID FL380 TFC RESULTS IN AIRSPD LOSS AND NEED TO DSND IN ORDER TO AVOID STALL.

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I WAS THE PF ON A FLT FROM DEN-AUS. WHILE AT APPROX FL350; ATC REQUESTED A CLB TO FL390 IN 3 1/2 MINS. WE ACCEPTED THE CLRNC. AT FL380; WE ENCOUNTERED A STRONG MOUNTAIN WAVE AND MODERATE TURB. AT FL387; OUR CLB RATE WAS ZERO. ATC ASKED IF WE WERE LEVEL AT FL390; AND WE ADVISED ATC THAT WE WERE NOT; AND REQUESTED A LOWER ALT. WE IMMEDIATELY ADVISED ATC OF THE SIT. ATC DENIED THE REQUEST AND DEMANDED US TO BE LEVEL AT FL390 IN 15-20 SECONDS. AFTER LEVELING AT FL390; AIRSPD STARTED TO DECREASE RAPIDLY DUE TO THE STRONG MOUNTAIN WAVE AND THE TURB. TO MAINTAIN A SAFE AIRSPD; WE HAD TO DEVIATE FROM FL390 AND LOSE ALT. WE MADE EVERY EFFORT TO KEEP ZDV INFORMED OF OUR FLT CONDITIONS AND PROMPTLY REQUESTED AMENDED CLRNC IN THE INTEREST OF SAFETY. WE RECEIVED A CLRNC TO FL370 SHORTLY AFTER. WE DID CONTINUE TO EXPERIENCE LIGHT TO MODERATE TURB ALL THE WAY TO AUS. I FEEL THE CAUSES OF THIS EVENT WERE ATC ISSUING A STEEP CLB GRADIENT; AS WELL AS ME ACCEPTING THE CLRNC. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR QUITE PROPERLY ASSUMED RESPONSIBILITY FOR ACCEPTING THE CLRNC. HE STATED THAT THEY DID; IN FACT; CONSULT THE ACFT PERFORMANCE PAGES TO ENSURE THAT FL390 WAS VIABLE. IT WAS THE TIME CONSTRAINT WHICH PROVED TO BE THE PROB. DUE TO THE ONCOMING TFC AT FL380; IT WAS IMPERATIVE THAT FL390 BE ACHIEVED IN A TIMELY FASHION. TO DO SO; THE ACFT WAS FORCED TO CLB AT THE EXPENSE OF AIRSPD AND WHEN FINALLY AT FL390; THEY WERE UNABLE TO SUSTAIN THE ALT DUE TO INSUFFICIENT THRUST TO ACCELERATE OR EVEN AVOID CONTINUED AIRSPD DETERIORATION. RPTR STATED HE WAS LESS PROACTIVE THAN HE SHOULD HAVE BEEN WHEN HE CONTINUED TO 'REQUEST' LOWER ALTS RATHER THAN DECLARING AN EMER AND DOING WHAT WAS NECESSARY TO REGAIN CTL OF THE ACFT. SUPPLEMENTAL INFO FROM ACN 652471: AFTER BEGINNING THE CLB; THE CAPT INCREASED THE RATE OF CLB BY REDUCING SPD FROM NORMAL CLB (.74 MACH). AT THIS POINT; OUR TCASII GAVE AN RA FOR THE XING TFC AT FL380. AFTER PASSING THE CONFLICTING TFC; ATC ISSUED A NEW ALT OF FL370 AND WE DSNDED NORMALLY.