A B757-200 IN CRUISE AT FL390 EXPERIENCED THE CABIN PRESSURE WARNING HORN AND REQUESTED A DSCNT TO FL330 WHERE THE CABIN RETURNED TO NORMAL.

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CABIN ALT ALERT DUE TO CABIN ALT AT 10000 FT. USUAL CABIN ALT AT CRUISE IS 7000-8000 FT. I DID NOT NOTICE THAT THE CABIN HAD BEGUN A SLOW INCREASE. DSCNT WITH OXYGEN MASKS ON WAS INITIATED TO FL330 AND CABIN RETURNED TO NORMAL IN APPROX 10 MINS. FLT TO DEST WAS UNEVENTFUL (ATL-DEN). MAINT WAS NOTIFIED AND INDICATED TO US THAT A CRACKED HEAT EXCHANGER COULD BE THE CAUSE. HOWEVER; NO SUCH PROB WAS FOUND IN DEN. MAINT INDICATED FURTHER CHKS WOULD BE UNDERTAKEN IN ATL AFTER THE RETURN FLT (DEN-ATL). CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE CREW WAS SURPRISED WHEN THE CABIN PRESSURE WARNING HORN SOUNDED AND A LOOK AT THE CABIN PRESSURE INDICATORS SHOWED THE CABIN WAS CLBING VERY SLOWLY. THE RPTR SAID ATC WAS ADVISED AND THEY WERE GIVEN FL330. BEFORE THIS ALT WAS REACHED THE CABIN PRESSURE HAD RETURNED TO NORMAL. THE RPTR STATED THE PRESSURIZATION SYS REMAINED IN AUTO AND OPERATED NORMALLY TO DEN WHERE MAINT ACCOMPLISHED A TEST OF THE SYS AND NO FAULTS WERE FOUND. THE RPTR SAID THE ACFT WAS ROUTED BACK TO ATL AND THE PRESSURIZATION OPERATED NORMALLY IN ALL MODES.