FLT CREW OF MD80 UNABLE TO MAINTAIN CRUISE MACH AT FL320. FORCED TO DSND TO AVOID STALL.

Narrative

APCHING PNG VOR WITH ZHU; FL320; RAM AIR TEMP -1 DEG; WT APPROX 138000 LBS; ON TOP OF CLOUDS; MACH .76; MACH ATL. NOTICED LIGHT AIRFRAME VIBRATION LIKE LNDG LIGHT OR SPOILER PANEL OUT. COMMENTED THIS TO FO. WE BOTH DID A QUICK CHK; GEAR DOORS; MCP; SPOILERS AND ICE DETECTION RINGS. SHUDDER BECAME MORE PRONOUNCED; AIRSPD DECLINING. I CALLED FOR ENG AND AIRFOIL ANTI-ICING. FO CONTACTED ZHU FOR FL260. WE WERE CLRED AND I DISCONNECTED AUTOPLT AND AUTOTHROTTLES; AND BEGAN A 1000 FPM DSCNT WITH MCL SELECTED MANUALLY. AT FL315; GOT 1 STALL AURAL AND 1 CLAXON/RED LIGHT. WE NEVER GOT A SPD LOW OR STICK SHAKER. AIRSPD NOW ACCELERATING IN A CONTINUED CTLED DSCNT. COULD HAVE LEVELED AT FL290 WITH PLENTY OF AIRSPD; BUT CONTINUED TO FL280 AND MACH .76 WITH ENG AND AIRFOIL ANTI-ICE ON. I FEEL THAT PERHAPS WE HAD PICKED UP ENOUGH ICE IN THE CLB TO GIVE A FLAWED EPR RATING; WITH LESS THRUST THAN ATL WOULD HAVE NORMALLY PROVIDED. AT NO TIME WERE THERE VISUAL SIGNS OF ICING EITHER BELOW THE WIPER OR ON THE ICE DETECTION RING EITHER SIDE!