What was found
AFTER START UP, THE PILOT WAS STARTING HIS TAXI ROLL WHEN THE ACFT BEGAN TO TURN LT. HE MADE SEVERAL ATTEMPTS TO CORRECT IT BUT THE ACFT CONTINUED LT UNTIL IT WAS HARD OVER. THE ACFT WAS BROUGHT IN AND THE PROBLEM COULD NOT BE DUPLICATED. THE POTS FROM THE ACTUATOR AND RUDDER PEDALS WERE REMOVED AND OHMED OUT. THE WIRING AND CONNECTORS WERE INSPECTED AND CHECKED OK. THE NWS SERVO VALVE WAS REMOVED AND SWAPPED WITH A KNOWN SERVICEABLE UNIT FROM ANOTHER ACFT AND BOTH OPS CHECKED OK. THE FOLLOWING DAY THE SUSPECT VALVE WHICH HAD BEEN INSTALLED ON ANOTHER ACFT FAILED WITH A UNCOMMANDED HARD LT TURN WHILE TAXIING. THERE APPEARS TO BE AN INTERMITTENT PROBLEM WITH THE SERVO VALVE SO A NEW VALVE WAS PURCHASED AND INSTALLED WHICH OPS CHECKED GOOD.