What was found
(CAN) HEADING INDICATION ON LT PRIMARY FLIGHT DISPLAY DRIFTING 7-8 DEGREES INTERMITTENTLY IN CRUISE FLIGHT AND ALSO NOTED ON THE GROUND. HEADING REPORTED TO DRIFT SLOWLY AND RETURN TO NORMAL A SHORT TIME LATER. ELECTRONIC STANDBY, MAGNETIC STANDBY AND RT PRIMARY FLIGHT DISPLAY HEADINGS WERE REPORTED AS MATCHING AND STEADY WHILE NR 1 DRIFTING. FAULT HISTORY DATABASE FILES FORWARDED TO MFG FIELD SERVICE, RECOMMENDATION WAS TO REPLACE THE ADAHRS. UNIT REPLACED, ON GROUND TESTS SATISFACTORY.