EMB145 CREW HAD VIOLENT ACFT SHAKING AND PITCH OSCILLATIONS. THE CTL YOKE WAS OPERATED IN A VIOLENT BUT DISTINCTIVE CYCLIC PATTERN. DISCONNECTING THE AUTOPLT STOPPED THE SHAKING.

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PITCH OSCILLATIONS WHEN WE REACHED CRUISE ALT. WE ENCOUNTERED LIGHT OCCASIONAL CHOP. AUTOPLT WAS FLYING WHEN ALL OF A SUDDEN THE ACFT STARTED TO SHAKE VIOLENTLY. THE NOSE OF THE ACFT FELT LIKE IT WAS BOUNCING UP AND DOWN WITHOUT ANY ROLL MOVEMENT AND THE CTL COLUMN WAS NOW MOVING BACK AND FORTH IN A VERY VIOLENT BUT DISTINCTIVE CYCLIC PATTERN. AT THIS POINT; I REALIZED IT WAS NOT TURB AND DISCONNECTED THE AUTOPLT AND IT IMMEDIATELY STOPPED. AT THIS TIME WE WERE STILL AT CRUISE AND IN SOLID IMC WHEN THE CAPT SUGGESTED WE TURN THE AUTOPLT BACK ON AND TRY IT OUT. SINCE I WAS FLYING; I TOLD HIM THAT I WOULD LIKE TO HAND FLY AND NOT TRY THE AUTOPLT. HE INSISTED THAT HE HAD HEARD OF A PREVIOUS SUCH INCIDENT AND WE WERE SUPPOSED TO TRY IT OUT. I INFORMED HIM NOT WHILE I WAS FLYING IN IMC. ONCE WE DSNDED BELOW 10000 FT; AND WE WERE BTWN LAYERS AND HAD INTERMITTENT GND CONTACT; THE CAPT TOLD ME THAT HE WOULD LIKE TO TRY RE-ENGAGING THE AUTOPLT AGAIN. I TOLD HIM ONCE MORE NOT WHILE I WAS FLYING AND I WAS UNCOMFORTABLE DOING THAT AT THAT POINT. ONCE IN PHL; CALLED MAINT OPERATIONAL CTLR AND CREW MEL'ED THE AUTOPLT.