AN MD80 IN CRUISE AT FL330 EXPERIENCED AN UNCOMMANDED PWR REDUCTION. BELIEVED BY CREW AND DEADHEADING PLT TO HAVE BEEN A COMPRESSOR STALL.

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WE WERE CRUISING AT FL330 JUST PASSING FAM IN LIGHT ENOUGH CIRRUS THAT WE COULD SEE THE CLOUD DECK BELOW US. WE WERE AT MACH PT 76; ENG AND AIRFOIL ANTI-ICE OFF. OUR FIRST INDICATION OF SOMETHING WRONG WAS THE FEEL OF DEVELOPING MODERATE CHOP WHICH BECAME MORE UNIFORM. THE CAPT; WHO WAS FLYING THE ACFT; THEN NOTICED OUR AIRSPD HAD DECREASED TO APPROX 220 KIAS. HE IMMEDIATELY DISCONNECTED THE AUTOTHROTTLE AND AUTOPLT; PUSHED UP BOTH THROTTLES AND INITIATED A VERY SLIGHT DSCNT. IMMEDIATELY AFTER HE MENTIONED OUR AIRSPD; I PLACED BOTH THE ENG AND AIRFOIL ANTI-ICE SYS ON AND GOT A CLRNC FROM CTR TO DSND TO FL290. CTR APPROVED THE DSCNT WITH NO DELAY AND ASKED US IF ALL WAS OK AND I RESPONDED IN THE AFFIRMATIVE. THAT WAS THE EXTENT OF CTR'S INQUIRY. EVERYTHING WAS BACK TO NORMAL WELL BEFORE LEVELING AT FL290. AT THAT POINT THE CAPT MADE A PA TO PAX AND ASKED THE FLT ATTENDANTS WHAT THEY HAD FELT IN THE BACK OF THE AIRPLANE. THE FLT ATTENDANTS SAID THEY FELT NO COMPRESSOR STALL; BUT ON DEPLANING; A DEADHEADING PLT SAID HE FELT A SLIGHT COMPRESSOR STALL IN ONE OF THE ENGS. THEREFORE; THE CAPT WROTE IT UP AND CALLED MAINT TO INVESTIGATE. SUPPLEMENTAL INFO FROM ACN 582149: WE FELT WHAT WE FIRST THOUGHT WAS LIGHT TO MODERATE CHOP DEVELOPING. IT WAS DIFFERENT FROM MOST CHOP IN THAT IT WAS MORE UNIFORM IN STRENGTH. FEELING THAT I IMMEDIATELY LOOKED AT THE AIRSPD INDICATOR. AIRSPD HAD UNKNOWINGLY DECAYED TO APPROX 220 KIAS. R ENG N1 WAS FLUCTUATING ABOUT +/-6 RPM'S AND EGT WAS NOT ELEVATED. I IMMEDIATELY PERCEIVED THAT WE WERE POSSIBLY APCHING A LOW SPD BUFFET. HE THEN CLRED US TO FL290 AND ASKED IF WE NEEDED FURTHER ASSISTANCE. BY THAT TIME THE BUFFET HAD STOPPED AND AIRSPD HAD INCREASED. THE BUFFETING HAD STOPPED AT APPROX FL326. THE ONLY CONCLUSION I CAN COME UP WITH IS THAT WE MUST HAVE BEEN IN ACTUAL ICING CONDITIONS AND I MISPERCEIVED IT; AND ALTHOUGH THE AUTOTHROTTLES DID NOT DISENGAGE I SHOULD HAVE NOTICED THE UNCOMMANDED PWR REDUCTION WHILE WE WERE CONVERSING.