As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CAPT ALTIMETER INDICATING 300' LOW DURING CLIMB AFTER 18,000' AT FL340 AND AFTER DESCENT TO FL280. ERROR WENT AWAY AFTER 15-20 MINUTES F/O & STBY ALT INDICATED SAME READING. REMOVED AND REPLACED CAPTAINS AIR DATA MODULE PER AMM 34-26-03-4 OPS CHECK GOOD A/C OK TO CONTINUE