MLG FLC HAS LOSS OF PRESSURIZATION AT FL370. ON DSCNT THEY WERE ABLE TO MAINTAIN PRESSURE AND RETURNED TO FL330.

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AT CRUISE FL370 THE CABIN ALT WARNING HORN CAME ON. CABIN ALT WAS 10000 FT AND CLBING 300 FPM. WE RAN THE LOSS OF PRESSURIZATION CHKLIST AND TRIED TO REGAIN CTL OF THE CABIN. THE OUTFLOW VALVE WAS CLOSED; DUCT PRESSURE NORMAL; 2 PACKS AND 2 BLEEDS OPERATING NORMALLY. WE COULD SEE NO REASON FOR THE PROB. WE TURNED THE PACKS ON HIGH. AS WE DSNDED WE OBSERVED THE CABIN AGAIN BEGIN TO PRESSURIZE. WE REQUESTED A LEVEL OFF WITH ATC AT 19000 FT AND ALLOWED THE CABIN TO STABILIZE AND ASKED FOR A CLB BACK TO ALT OF 33000 FT. THE SYS OPERATED NORMALLY IN ALL MODES. AFTER LNDG IN CLEVELAND AND TALKING TO MAINT WE DISCOVERED THE FORWARD CARGO DOOR SLIGHTLY AJAR AND A LINE OF ICE BTWN THE DOOR AND FUSELAGE. I SUSPECT THERE WAS A PROB WITH THE DOOR NOT CLOSING ALL THE WAY BUT ENOUGH WHERE WE DIDN'T GET A LIGHT (WARNING). I BELIEVE THE PRESSURE DIFFERENTIAL AT FL370 WAS ENOUGH TO PUSH THE DOOR (WHICH WAS SLIGHTLY SPRUNG OR NOT CLOSED TIGHT) OUT ENOUGH TO CAUSE A SLOW LEAK. IT SEALED OK AT FL330 AND BELOW. THIS IS MY THEORY AS I WAS UNABLE TO FOLLOW UP WITH MAINT.