A DC9-30 STARTS LOSING CABIN PRESSURE AT A SLOW RATE. CREW HAD STARTED A CTLED DSCNT TO 16000 FT WHERE THE CABIN WAS STABILIZED. CABIN ATTENDANT TOLD THE CAPT THAT THE PAX NEVER NOTICED MUCH OF A PROB EXCEPT FOR SOME EAR DISCOMFORT. THIS ACFT HAD A SIMILAR PROB ON THE PREVIOUS LEG; AND THE PRESSURE CTLR HAD BEEN CHANGED.

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WHILE WE WERE CRUISING AT FL330 THE FO NOTICED THAT THE CABIN WAS CLBING AT 500 FPM. OUR FIRST ACTION WAS TO ENSURE THAT THE PRESSURE CTLR WAS CLOSED FULL FORWARD IN AUTO -- IT WAS; BUT THE CABIN CONTINUED TO CLB AT 1100 FPM NOW. WE THEN REQUESTED A LOWER ALT FROM CTR AND THEY INITIALLY GAVE US FL240 FOLLOWED SHORTLY THEREAFTER WITH CLRNC TO 16000 FT. SHORTLY AFTER STARTING OUR DSCNT THE 'CABIN PRESSURE' WARNING LIGHT ILLUMINATED; INDICATING THAT THE CABIN ALT HAD GONE ABOVE 10000 FT. WE DONNED OUR OXYGEN MASKS AND I CALLED FOR THE 'CABIN PRESSURE LOSS/RAPID DSCNT' CHKLIST. SOMEWHERE BTWN FL230 AND FL200; THE CABIN STARTED DSNDING AT 2000 FPM. THE CABIN PRESSURE CTLR; WHICH HAD BEEN PLACED TO FULL CLOSED AS INSTRUCTED BY 'CABIN PRESSURE LOSS' CHKLIST; WAS RETURNED TO AUTO. THE CABIN NOW STABILIZED AND WE LEVELED OFF AT 16000 FT. ABOUT THE TIME THE CABIN WAS STABILIZING THE LEAD FLT ATTENDANT CAME TO THE COCKPIT BECAUSE SHE HAD NOTICED SOME EAR DISCOMFORT. WE INFORMED HER OF THE SIT AND SHE TOLD US THAT EXCEPT FOR NOTING SOME EAR DISCOMFORT; ALL WAS FINE IN THE CABIN WITH THE PAX. AFTER CONFERRING WITH COMPANY MAINT AND DISPATCH; WE DECIDED TO CONTINUE TO OUR DEST AT FL240. THIS ACFT ON A PREVIOUS LEG WITH A DIFFERENT CREW HAD THE SAME PROB AND WAS WRITTEN UP IN THE MAINT LOG. OUR MECHS TESTED THE ACFT FOR LEAKS AND CHANGED THE AUTO CABIN PRESSURE CTLR. I FEEL THAT IF WE HAD GOTTEN MORE INFO FROM THE PREVIOUS CREW IN THE WRITE-UP AND/OR A DEBRIEFING; WE WOULD HAVE BEEN BETTER ABLE TO DETERMINE IF REPLACING THE AUTO PRESSURE CTLR WAS THE APPROPRIATE FIX. OBVIOUSLY; IN HINDSIGHT; IT WAS NOT.