As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT FL 380 PRESSURIZATION WAS AT MAX DIFFERENTIAL PSI. CABIN ALT WAS 7500' APPROX, AND CABIN VSI WAS CYCLING +/- 700 TO 800 FPM CONSTANTLY, DESCENDED TO FL 360 CABIN ALT STAYED ABOUT 7500, BUT DIFFERENTIAL WAS APPROX 7-8 PSI, COBIN VSI STEADIED OUT. NO ADVERSE OUTCOMES. GROUNDED - YES. REMOVED AND REPLACED HIGH STAGE VALVE REF B737-900 AMM 36-41-06.