THE RIGHT ENGINE WAS STARTED AS THE PASSENGERS COMPLETED BOARDING. AFTER THE COPILOT SECURED THE CABIN DOOR AND TOOK HIS SEAT, THE PILOT RELEASED THE PARKING BRAKE AND CONTROL LOCK AND APPLIED POWER TO TAXI. WHEN A TURN WAS COMMANDED THE NOSEWHEEL STEERING DID NOT RESPOND AND THE PILOT APPLIED NORMAL AND EMERGENCY BRAKING WITHOUT EFFECT ON THE AIRCRAFTS MOVEMENT. THE PILOT THEN ORDERED THE RIGHT ENGINE SHUT DOWN. THE AIRCRAFT CONTINUED TO ROLL ACROSS THE RAMP UNTIL IT COLLIDED WITH A PARKED AIRCRAFT. THE OPERATOR HAD EXPERIENCED SIMILAR FAILURES DURING THE 6 MONTHS PRIOR TO THIS INCIDENT. HOWEVER, ALL EFFORTS TO DUPLICATE AND ISOLATE THE PROBLEMS HAD BEEN UNSUCCESSFUL. DURING POST INCIDENT REPAIRS, NUMEROUS LANDING GEAR SYSTEM COMPONENTS WERE REPLACED AND THE PROBLEM HAS NOT SURFACED AGAIN. NO REASON FOR THE ORIGINAL FAILURES WAS DETERMINED.