ACFT EQUIP PROB. ACR LOST PRESSURIZATION ON CLBOUT AND RETURNS TO LAND.

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ON DEP FROM LGA AFTER CLEANUP CHKS AND 10000 FT CHKS; APCHING 18000 FT; CABIN ALT WARNING CAME ON WITH THE ASSOCIATED AURAL. CABIN ALT WAS OBSERVED AT 11000 FT AND CLBING RAPIDLY. AN EMER DSCNT WAS INITIATED AND AN EMER DECLARED. AN IMMEDIATE CLRNC TO 10000 FT WAS REQUESTED AND RECEIVED. IN THE DSCNT THE PRESSURIZATION CTLR SEEMED TO CATCH AND THE CABIN STARTED TO REPRESSURIZE. MANUAL CTL WAS NOT INITIATED BECAUSE OF THIS. UPON TALKING WITH THE AFT FLT ATTENDANT; SHE COMPLAINED ABOUT VERY LOUD NOISE COMING FROM THE AFT GALLEY DOOR. I BECAME CONCERNED THEN THAT WE HAD AN OPEN DOOR; ALTHOUGH THERE WERE NO DOOR WARNING ANNUNCIATORS ON. WE STAYED WITH THE DECLARED EMER AND RETURNED TO LGA. ON DSCNT THROUGH 12000 FT; THE ATC CTLR ASKED TO LEVEL AT 12000 FT. SINCE WE HAD THE APPARENT LACK OF STRUCTURAL INTEGRITY AND SINCE THE PRESSURIZATION CTLR WAS RELEASING CABIN PRESSURE AGAIN; I INSISTED ON CONTINUING THE DSCNT TO 10000 FT. WE CONTINUED TO 10000 FT WITH THE CONCURRENCE OF ATC AND CAME APPROX WITHIN 2 MI OF ANOTHER ACFT AT 11000 FT. NO CONFLICT WAS RPTED BY ATC OR THE INVOLVED ACFT. LNDG WAS UNEVENTFUL AT LGA AND THE CRASH EQUIP WAS RELEASED AFTER WE CLRED THE RWY. LNDG WAS ACCOMPLISHED AT 129700 LBS GROSS WT. THE CTLR WAS REPLACED BY MAINT AND WE WERE RE- RELEASED BY DISPATCH FOR THE FLT TO DFW.