As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FMC DROPPED WPT NAVIGATING TO ON CLIMB OUT RE ENTERED AND ACTIVATED ROUTE 2 WPT NAV DROPPED A SECOND TIME. CREW ELECTED TO PERFORNM AN AIR TURNBACK.AIRCRAFT LANDED WITHOUT ANY FURTHER INCIDENCE. AIRCRAFT WAS GROUNDED BY MAINTENANCE. MAINTENANCE FOUND EXCESSIVE DRIFT FOR LEFT IRS. REPLACED THE LEFT IRS REF B767 AMM 34-21-00. VERIFIED PROPER IRS FUNCTION ALONG WITH FMS SYSTEMS, REF B767 AMM 34-61-00. AIRCRAFT RETURNED TO SERVICE