BE60 PILOT REPORTS AUTOPILOT PITCH UP ON ENGAGEMENT AND DIFFICULTIES FLYING ACFT AFTER DISENGAGEMENT. EMERGENCY DECLARED AND SUCCESSFUL RETURN TO DEPARTURE ARPT.

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AT ASSIGNED CRUISE ALT; I PRESSED THE ALT HOLD BUTTON ON AUTOPLT. THE AUTOPLT COMMANDED A VERY SHARP PITCH UP; I DISENGAGED THE AUTOPLT AND STOPPED THE CLB. THE AUTOPLT DISENGAGED BUT I BELIEVE THE CLUTCHES ON THE SERVOS DID NOT RELEASE; DUE TO VERY HVY CTL FORCES DURING INCIDENT. DUE TO THE CTL FORCES AND IMC CONDITIONS; CTL OF ACFT WAS MOMENTARILY IN QUESTION. I ASKED FOR AND RECEIVED CLRNC FROM CTR TO RETURN TO ZZZ DUE TO DETERIORATING WX AT DEST. UPON COMPLETION OF TURN AND STABILIZATION OF AIRPLANE I TRIED TO RELIEVE THE CTL PRESSURES BY ACTIVATING THE PITCH COORDINATOR AND THE ACFT WENT INTO AN UNCOMMANDED CLB AGAIN. I DISENGAGED AND MOMENTARILY CTL WAS IN QUESTION AGAIN; AND THIS TIME I BUSTED IFR ALT ASSIGNMENT; I DECLARED AN EMER WITH APCH. I WAS GRANTED; AND I COMPLETED AN UGLY BUT SUCCESSFUL GPS APCH INTO ZZZ AND RETURNED ACFT FOR AUTOPLT REPAIR. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: MAINTENANCE FOUND THAT THE BAROMETRIC ALTITUDE SENSOR HAD FAILED CAUSING THE INITIAL PITCH UP AND A LOOSE WIRE PREVENTED THE AUTOPILOT FROM BEING DISENGAGED. THE REPORTER WAS ABLE TO OVERPOWER THE AUTOPILOT BUT THE EXPERIENCE WAS VERY DISCONCERTING IN IMC.