AN A320 ACFT ENCOUNTERED MOUNTAIN WAVE; DECELERATED; AND BEGAN DESCENDING WITH MAX CONTINUOUS THRUST BEFORE ATC DESCENT CLRNC WAS RECEIVED.

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FLYING IN THE VICINITY OF LAA VOR; WE HAD JUST CLBED TO FL350 ATTEMPTING TO CLR CLOUDS AND FIND A BETTER RIDE. WE BEGAN TO ENCOUNTER MODERATE MOUNTAIN WAVE ALONG WITH LIGHT TURB AND OUR AIRSPD WAS FLUCTUATING +/-25 KTS. WHILE WE WERE BELOW THE RECOMMENDED MAX ALT FOR OUR WT; AND WELL WITHIN THE 1.3 G BUFFET ENVELOPE; WE WERE UNABLE TO MAINTAIN A SAFE SPD AND ADVISED ATC WE WERE UNABLE TO MAINTAIN ALT AND THAT WE WERE BEGINNING A DSCNT. WE BEGAN A DSCNT OUT OF FL350 AND RECEIVED A CLRNC TO FL330 WHILE DSNDING THROUGH FL344. WE STARTED THE DSCNT BEFORE BEING ISSUED A CLRNC BECAUSE THE ACFT HAD SLOWED BELOW WHAT WE CONSIDERED TO BE A SAFE SPD (VLS 10/15 KTS) EVEN WITH MAX CONTINUOUS THRUST BEING APPLIED. THE ACFT WAS COMING DOWN WITH OR WITHOUT A CLRNC. THERE WERE NO TARGETS CLOSE BY ON THE TCAS AND THERE WAS NO CONFLICT WITH OTHER TFC. IN 27 YRS OF FLYING JET ACFT; THIS HAS NEVER HAPPENED TO ME. WE DID EVERYTHING RIGHT. WE WERE NOT TRYING TO FLY THE ACFT BEYOND ITS CAPABILITIES. WE WERE WELL WITHIN THE PROPER FLT ENVELOPE. IT'S JUST ONE OF THOSE THINGS THAT CAN HAPPEN. MOUNTAIN WAVE AND TURB CAN PUT YOU OUTSIDE THE SAFE ENVELOPE IN A HURRY. WINDS ALOFT WERE UNUSUALLY STRONG. YOU DON'T USUALLY ENCOUNTER WAVES THAT STRONG THIS LATE IN THE SEASON.