What was found
(CAN) AIRCRAFT ORIGINALLY HAD PRESSURIZATION PROBLEMS, SYSTEM WAS TESTED AND OUTFLOW VALVE WAS FOUND TO BE DEFECTIVE. A CONTROLLER WAS REMOVED FROM AIRCRAFT IN THE SHOP AND INSTALLED IN AIRCRAFT C-GFLA FUNCTIONAL TESTS WERE CARRIED OUT AND SYSTEM FUNCTIONED NORMAL. WHEN A NEW CONTROLLER WAS ORDERED AND INSTALLED THE PILOTS REORTED THE (CABIN)ALTITUDE WAS FLUCTUATING. A SECOND CONTROLLER WAS ORDERED. INSTALLED AND FUNCTIONAL TESTED NORMAL. PILOTS REPORTED NO PROBLEMS WITH LAST CONTROLLER.