AN A320 CREW; DURING CRUISE AT FL370; ENCOUNTERED A MODERATE TO SEVERE MOUNTAIN WAVE; CAUSING AN ALT VARIATION OF PLUS 350 FT TO MINUS 700 FT; ACCOMPANIED BY A MOMENTARY SPD INCREASE TO VMO PLUS 3.

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CAPT WAS FLYING LEG FROM SFO TO DEN. WE HAD RPTS OF LIGHT CHOP AT FL370. ABOUT 40-50 NM W OF HVE WE ENCOUNTERED LIGHT TO MODERATE TURB. WE TURNED ON THE SEATBELT SIGN AND TOLD THE FLT ATTENDANTS TO BE SEATED. WE WERE HAVING A DISCUSSION ABOUT THE EFFECT OF MOUNTAIN WAVE ON THE AUTOPLT AND THE CAPT BUGGED THE SPD BACK TO AROUND .76 MACH WHEN WE ENCOUNTERED MODERATE TO SEVERE MOUNTAIN WAVE. THE CAPT DISCONNECTED THE AUTOPLT AND REDUCED THE THROTTLE POS; BUT THE SPD STILL INCREASED TO APPROX VMO +3 KTS FOR A FEW SECONDS. WHILE TRYING TO GET THE AIRSPD BACK BELOW VMO; THE AIRPLANE CLBED APPROX 350 FT. THEN THE AIRSPD DECAYED RAPIDLY AND THE AIRPLANE LOST APPROX 700 FT. I RECOMMEND TO THE CAPT TO DISCONNECT THE AUTOTHROTTLES; SET HIS PWR AND TRY TO STABILIZE THE AIRPLANE. I ALSO CONTACTED ATC AND NOTIFIED THEM OF THE DEV FROM ALT DUE TO THE MOUNTAIN WAVE. HE SAID THERE WAS NO TFC CONFLICT AND ASKED US TO DO OUR BEST TO RETURN TO OUR ASSIGNED ALT ASAP. THE CAPT DID A GOOD JOB OF MAINTAINING CTL GIVEN THE CIRCUMSTANCES. WE MADE A LOGBOOK WRITE-UP FOR THE OVERSPD AND NOTIFIED DISPATCH OF OUR ENCOUNTER AS SOON AS THE AIR SMOOTHED OUT.