What was found
CABIN PRESSURE FLUCTUATES +0.5, UP TO -0.3, DN, PSI CONTINUOUSLY IN CRUISE FLIGHT IN AUTO MODE, FL380, AND ALTN MODE. ACFT GROUNDED. DID FIM 21-31, TASK 801. BITE CK OF BOTH DCPC. NO FAULTS FOUND. PRESSURIZED ACFT WITH APU TO 3 PSI DIFF, ACFT HELD PRESSURE. TECH SERVICES REQUESTED TO REPLACE OUTFLOW VALVE. R & R AFT OUTFLOW VALVE ASSY REF AMM 21-30-03-000-801 AND AMM 21-31-03-400-801. INSTALLATION TEST GOOD.