FLC OF A SAAB SF340B (SF34-L); DURING CRUISE LEVELOFF; AND BREAKING OUT ON TOP OF AN OVCST; WHERE ICING CONDITIONS EXISTED; THE FO FLYING EXPERIENCED ACFT SHAKING; VIBRATION AND INVOLUNTARY WING DROP; CAUSING A MOMENTARY LOSS OF ACFT CTL. THE ACFT WAS IMMEDIATELY DSNDED TO A LOWER ALT WHERE IT STOPPED SHAKING AND NORMAL CTL FEEL RETURNED.

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WHILE LEVELING OFF AT 16000 FT THE AIRPLANE BEGAN TO SHAKE AND VIBRATE. THE STALL SHAKER SOUNDED AND THE L WING DROPPED. THE ACFT HAD SOME LIGHT MIXED ICE ON THE WINGS. WE TOLD ATC WE NEEDED LOWER AND DSNDED TO 15000 FT. EVEN AFTER OPERATING THE DEICE BOOTS; THE AIRPLANE'S ROLL AND PITCH CTL WAS VERY SLUGGISH. WE EXITED ICING CONDITIONS AND THE ACFT PERFORMED NORMALLY. WE BELIEVE THE FORMATION OF ICE ON THE WINGS AND TAIL WAS NOT SMOOTH AND UNIFORM; WHICH DISTURBED THE AIRFLOW. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THEY HAD JUST BROKEN OUT OF THE OVCST ON TOP AND STARTED LEVELING AT 16000 FT WHEN THE ACFT STARTED TO SHAKE; VIBRATE AND FALL OFF ON 1 WING. ONLY A SMALL AMOUNT OF ICE WAS OBSERVED ON THE WINGS. THE STICK SHAKER ALSO STARTED VIBRATING AS THE NOSE WAS LOWERED FOR LEVEL FLT. IT WAS NOT UNTIL THE ACFT WAS DSNDED AND BUILT UP FROM APPROX 140 TO 175 KTS THAT THE ACFT STOPPED VIBRATING AND THE CTLS STARTED FEELING NORMAL. AT 11000 FT; THE ACFT LEVELED OFF IN THE CLR AND WAS REACTING NORMAL. THE OBSERVED ICE ON THE ACFT WAS GONE ALSO. HE COMMENTED THAT HE HAD 5500 HRS IN THE SAAB 340; AND HAD FLOWN MANY TIMES IN ICING CONDITIONS; BUT HAD NEVER EXPERIENCED ANY ACFT CTL OR VIBRATION PROB. HE BELIEVED THIS TO BE AN ISOLATED INCIDENT SINCE THERE WAS SUCH A SMALL AMOUNT OF ICE OBSERVED. HE ALSO BELIEVED IT WAS NOT ADVISABLE TO TURN ON THE DEICE BOOTS FOR FEAR THAT IT WOULD CAUSE A 'BRIDGING' OF THE ICE LAYER AND THEREFORE; NOT REMOVE THE ICE. HE STATED THAT THE SHAPE OF THE SMALL AMOUNT OF ICE WAS SO ODD; LIKE A MUSHROOM; THAT IT CAUSED AN UNUSUAL AMOUNT OF DRAG. HE FURTHER STATED THAT THE ACFT'S REACTION WAS A MOMENTARY LOSS OF CTL BY THE LATERAL AND PITCH AXIS.