What was found
AT FL 380, CABIN RATE INDICATOR WENT FROM 1200 FPM CLIMB TO 2000 FPM DESCEND EVERY 2-3 SECONDS SWITCHING TO ALTERNATE AHD NO EFFECT, DESCEND TO FL 340 AND EVERYTHING BECAME NORMAL. ACFT GROUNDED. NR 1 AND NR 2 CABIN PRESSURE CONTROLLERS LAST LEG SHOWS LOW INFLT HIGH LEAK G FAULT CODE 030. AS A PRECAUTION R & R AFT OUTFLOW VALVE. OPS CHECKED GOOD REF MM 21-31-03. PERFORMED SYS TEST OF POSITIVE PRESSURE RELIEF VALVES IAW AMM 21-32-01. OTBD VALVE FAILS TEST. REPLACED INBD AND OTBD PRESSURE RELIEF VALVES IAW AMM 21-32-01. OPS CHECK NORMAL. NO OTHER DISCREPANCIES NOTED.