CORP ACFT HAS ICE BUILDUP ON EMPENNAGE. WHEN THEY TRY TO CLB; ACFT STALLS.

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WE WERE IFR TO EQY (MONROE; NC) LEVEL AT 11000 FT; CLRED DIRECT TO AHN DIRECT GRD; UNARMI ARR. WE WERE SOLID IMC; SMOOTH; OUTSIDE AIR TEMP ABOUT 0 DEGS C. THE ACFT WAS STABLE IN CRUISE. WE WERE NOTICING VERY LIGHT RIME ICE; BUT IT WAS NOT ACCUMULATING ON THE WINGS. THE CLOUDS ABOVE LOOKED 'LIGHTER;' SO WE DECIDED TO REQUEST 13000 FT AND WE WERE CLRED UP TO 13000 FT. UPON INITIATING THE CLB; THE ACFT PITCHED DOWN; BUFFETING. THE PIC ATTEMPTED TO REGAIN PITCH CTL AND THE STALL HORN WENT OFF. AT THIS POINT; THE PIC PUSHED UP THE PWR; AND I CONTACTED CTR TO ASK FOR THE NEAREST ARPT AND ADVISE THAT WE HAD A 'VIBRATION' AND WERE EXPERIENCING DIFFICULTY; WE WERE TOLD THAT ATHENS WAS CLOSEST AND WERE CLRED DIRECT. I TOLD THEM TO 'STAND BY.' THE PIC HAD THE ACFT IN CTL AND WE WERE LEVEL AT APPROX 9000 FT MSL. WE REQUESTED BACK UP TO 13000 FT MSL AND BACK ON COURSE. THE ACFT PERFORMED NORMALLY AND THE REST OF THE FLT WAS UNEVENTFUL. LOOKING BACK; I BELIEVE THAT; ALTHOUGH THE WINGS WERE RELATIVELY FREE FROM ICE; THERE MAY HAVE BEEN AN ACCUMULATION ON THE TAIL. THE ACFT WAS EQUIPPED WITH PNEUMATIC BOOTS; BUT THERE WAS NEVER ENOUGH ICE PRESENT (THAT I OBSERVED; AND I WAS WATCHING) TO CYCLE THE BOOTS EFFECTIVELY. SOMEHOW; THE FLOW AROUND THE EMPENNAGE WAS DISTURBED; AND THE PITCH UP TO INITIATE THE CLB TO 13000 FT FROM 11000 FT MUST HAVE STALLED THE HORIZ STABILIZER. THERE WAS NO PERCEPTIBLE TRIM CHANGE TO ALERT US TO THIS CONDITION; AND THE SUBSEQUENT EVENTS WERE VERY RAPID. THE PIC'S INITIAL RESPONSE TO THE DRAMATIC PITCH DOWN WAS TO INCREASE BACK PRESSURE; WHICH WAS A NATURAL REFLEX. HE SUBSEQUENTLY RELEASED BACK PRESSURE; PWRED UP; AND MADE A GOOD IMC STALL RECOVERY. WE QUICKLY RAN THROUGH A CLB CHKLIST AND HEADED FOR DRY AIR (WITH CLRNC). CTR WAS VERY HELPFUL.