As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT FL 400, CABIN ALTITUDE INDICATOR FLUCTUATED BETWEEN 20,000- 10,000 FT. NO CABIN ALTITUDE EICAS CAUTION MESSAGE OR MASTER CAUTIONS ILLUMINATED. DESCENDED TO FL 340 AND HAD NORMAL INDICATION. CABIN DIFFERENTIAL PRESSURE NEVER WENT INTO THE AMBER BAND. REMOVED AND REPLACED THE CABIN PRESSURE GAUGE PANEL ASSY IAW AMM 21-33-01. OPS CHECKED GOOD.