As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHILE IN CRUISE AT FL310 THE CABIN ALTITUDE BEGAN TO RISE AND THE CREW EXECUTED A RAPID DESCENT. THERE WAS NO ADVERSE PASSENGER REACTION AND LIKELY MOST OF THE PASSENGERS WERE UNAWARE. THE FLIGHT DEPARTED UNDER MEL RELIEF FOR A BLEED SYS INOP. THE DESCENT WAS FROM FL310 TO FL200 WHEN THEY GOT THE PRESSURIZATION UNDER CONTROL FOUND PRV FLUCTUATING DURING ENG RUN. REPLACED NR 2 PRESS REGULATING VALVE IAW AMM 36-11-52, PB.401. LEAK AND OPS CHECKED OK.