As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CREW REPORTED CABIN PRESSURE POPPING AND VERY UNCOMFORTABLE ON CLIMBOUT WITH APU OFF AND USING ENGINES AS BLEED SOURCE. LEFT PRESSURE REGULATING SHUTOFF VALVE CYCLING NONSTOP. CREW RETURNED TO FIELD AND LANDED WITHOUT INCIDENT. MAINTENANCE TROUBLESHOT, R & R LT ENGINE PRESSURE REGULATING SHUTOFF VALVE AND PERFORMED CHECKS IAW AMM. ACFT RELEASED FOR SERVICE.