What was found
CABIN OUTFLOW VALVE POSSIBLY STICKING. CABIN CLIMBED IN EXCESS OF 2,000 FT PER MINUTE, MANUAL CONTROL HAS NO EFFECT. CABIN WENT BACK TO NORMAL, 5 MINUTES LATER LOSS OF CABIN AGAIN, SAME CONDITIONS. CREW DID AN EMERGENCY DESCENT. MAINTENANCE TESTED OUTFLOW VALVES, AUTO PRESSURIZATION AND MANUAL PRESSURIZATION IN ACCORDANCE WITH BOEING MM CHAPTER 21-30-0. ALL SYSTEMS OPS CHECKS NORMAL. COULD NOT DUPLICATE.