As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
IN CRUISE WITH AUTOPILOT IN AUX NAV INS NR 1, AIRCRAFT WOULD SUDDENLY ROLL RIGHT AND DIVERT FROM COURSE. OPS OK ON SYSTEM NR 2. PERFORMED SYSTEM TEST. NR 1 INS SHOWED CODE 4 DURING ALIGNMENT. REMOVED AND REPLACED NR 1 INU. PERFORMED ROLL CHANNEL CHECK; FAILED CHECK S1-1. REMOVED AND REPLACED ROLL COMPUTER. AUTOPILOT AND INS OPS CHECKED OK IAW MM 22-12-1 AND SMM 34-44-0.