As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: PILOT REPORTS APPROXIMATELY EVERY 20 SECONDS THE CABIN PRESSURE FLUCTUATES CAUSING CABIN ALTITUDE TO CHANGE. CABIN VERT SPEED MOVES UP TO 400 FT/MIN. SYSTEM ACTS THE SAME ON BOTH SYSTEMS. MAINTENANCE FOUND L/H FLOW CONTROL VALVE 23HB AT FAULT. REMOVED AND REPLACED THE L/H FLOW CONTROL VALVE IAW AMM 21-51-51. OPS PLUS LEAK CHECK GOOD.