What was found
(CAN) AFTER TAKEOFF, A/C OVERPRESSURIZED DUE TO FAULTY NR SYSTEM AND NR SYSTEM DID NOT TAKE CONTROL. ALSO, DIFFICULTY ENCOUNTERED WITH MANUAL CONTROL. CLIMB-OUT OF FLIGHT LEVEL 320 LOST CONTROL OF CABIN, FIRST DESCENDED, THEN FULL CLIMB AT 3,700 FEET PER MINUTE, CONTROL REGAINED, OK IN MANUAL. A/C RETURNED TO DEPARTURE TERMINAL. MAINTENANCE ACTION INVOLVED CHANGING OUT FLOW VALVE ASSEMBLY AND OPERATIONALLY CHECKED SERVICEABLE.