As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
INTERMITTENT - ALT DISAGREE - CAUTION MESSAGE DURING FLIGHT. CAPT AND F/O ALTIMETERS AS MUCH AS 250 FT DIFFERENCE AT FL310. SEEMED TO IMPROVE SLIGHTLY DURING FLIGHT. MCDP SHOWED LAST FLT LEG, L-V NAV FAIL AD SYS L IRMP. SHOWED LT SYS FAULT CODE 34-21050. SWAPPED LT AND CTR IDIRUS. PERFORMED OPS CK OK PER FIM 34-26-00-101.