DC10-30 EXPERIENCES MOUNTAIN WAVE ACTIVITY AT FL390. UNABLE TO MAINTAIN ALT; THE ACFT RECEIVES VECTORS FOR AN OFF COURSE DSCNT AND HAS TO START DSCNT EARLY ON ACCOUNT OF DECAYING AIRSPD. CTR ASKED FOR A CALL ON GND.

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CRUISING AT FL350 IN AREA RPTED HAVING MOUNTAIN WAVE ACTIVITY BTWN FL260 AND FL340. WE REQUESTED A 'WRONG WAY CRUISE ALT' OF FL370; WHICH WAS GRANTED. JUST AFTER LEVELING AT FL370; CTR CTLR ASKED IF WE COULD CLB TO FL390. AFTER CHKING OUR PERFORMANCE MANUALS AND AGREEING THAT WE COULD HOLD FL390 WITH 1.3G PROTECTION; WE CLBED TO FL390. JUST AFTER REACHING FL390; WE WERE HIT BY STRONG MOUNTAIN WAVE ACTIVITY AND TURB. ACFT BUFFETED DUE TO LOW SPD STALL AND THEN WENT INTO MACH BUFFET. CREW WAS UNABLE TO CTL AIRSPD AND ALT AND IMMEDIATELY REQUESTED A LOWER ALT. CTLR HAD CONFLICTING TFC AND EITHER SAID 'STANDBY' OR 'IT'S NOT AVAILABLE.' WE ATTEMPTED TO HOLD ASSIGNED ALT BUT LOST APPROX 300 FT. CAPT WAS GETTING READY TO DECLARE AN EMER TO DSND; WHEN ATC GAVE US A RADAR VECTOR AND A DSCNT CLRNC TO A LOWER ALT. WE STARTED DOWN IMMEDIATELY. LOWER ALTS WERE ALSO AFFECTED BY MOUNTAIN WAVE ACTIVITY BUT WE HAD A WIDER STALL AND MACH BUFFET PROTECTION. A FEW MINS AFTER RECOVERING AT FL370; ATC REQUESTED THAT WE CALL ZLC TO EXPLAIN WHAT HAPPENED AND OUR ALTDEV. EVEN THOUGH WE WERE 5000 FT ABOVE THE FORECAST MOUNTAIN WAVE ACTIVITY; WE STILL ENCOUNTERED DIFFICULTY. SUPPLEMENTAL INFO FROM ACN 364560: GIVEN A 30 DEG R TURN TO FACILITATE LOWER. DURING TURN FORCED TO START SHALLOW DSCNT WITH MAX THRUST JUST TO MAINTAIN AIRSPD ABOVE STALL. EXPERIENCED HIGH SPD BUFFET 3 TIMES AND LOW SPD BUFFET ONCE DURING ENTIRE EPISODE THAT LASTED APPROX 30 SECONDS.