What was found
DURING CRUISE AT FL340, HAD AN UNSCHEDULED PRESSURIZATION CHANGE. CABIN STARTED TO DESCEND AT APPROXIMATELY 2000 FT PER MINUTE. ACCOMPLISHED QRH CHECKLIST, CABIN STABILIZED IN ALTERNATE MODE. CABIN DIFFERENTIAL 8.6-8.7, CABIN ALTITUDE 7,000 - 7,500 FT. ACFT GROUNDED. REMOVED AND REPLACED AFT OUTFLOW VALVE ASSEMBLY IAW AMM 21-31-03-000-801, AND 21-31-03-400-801, ALL TESTS PASSED.