A B747-400 IN CRUISE AT FL350 THE CREW ATTEMPTED TO TROUBLESHOOT A CHRONIC PRESSURIZATION SYS AND LOST CABIN PRESSURE FOR ABOUT 40 SECONDS.

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THE CABIN PRESSURIZATION RATE HAD BEEN FLUCTUATING 500 FPM THROUGH FLT AND WAS DOCUMENTED IN PREVIOUS WRITE-UPS. CAPT DECIDED TO TROUBLESHOOT AND CYCLED OUTFLOW VALVES INDIVIDUALLY TO MANUAL THEN BACK. UPON CYCLING ONE OF THE VALVES; THE OUTFLOW VALVES OPENED; CAUSING THE CABIN TO CLB IN EXCESS OF 9900 FPM. CABIN ALT REACHED AROUND 11400 FT; ACCORDING TO THE CAPT; BEFORE IT BECAME CTLABLE AGAIN IN MANUAL. THE OUTFLOW VALVES WERE CLOSED AND THE CABIN DROPPED BELOW 10000 FT; THEN THE PRESSURIZATION DIFFERENTIAL WENT TO 9.0. ONCE THE FLT MANUAL WAS CONSULTED; THE PRESSURE CONTROLLER WAS SELECTED TO 'A;' THE VALVES PLACED IN BACK INTO AUTO; AND NORMAL PRESSURE RESTORED. THIS ACTION WAS ALSO NOTED TO HAVE BEEN TAKEN BY A PREVIOUS CREW; IN A JAN/FRI/01 WRITE-UP. THE FO'S BECAME AWARE OF PROB WHEN THE CABIN BEGAN CLBING EXCESSIVELY. AT 10000 FT; ONE OF THE NON FLYING FO'S DIRECTED THE FLYING FO TO TAKE CTL OF THE ACFT. THE QRC WAS NOT EXECUTED; AS IT WAS FELT THAT THE CABIN ALT CTL WAS QUICKLY REGAINED. TIME FROM 6000 FT TO 11400 FT WAS ABOUT 20 SECONDS; AND TIME TO RESTORE BELOW 10000 FT WAS ABOUT 40 SECONDS. THE ENTIRE INCIDENT TOOK ABOUT 5 MINS. A DSCNT WAS INITIATED WITH ATC WHILE THE OUTFLOW VALVES WERE IN MANUAL; AND NORMAL OPS WERE RESTORED WITH THE CABIN CONTROLLER IN 'A.'