What was found
UNCOMMANDED YAW MOTION SHORTLY AFTER ENGAGING AUTOPILOT ABOVE 20,000 FT, NOSE SWUNG RIGHT. DISCONNECTED YAW DAMP AUTOPILOT, ACFT SWING BACK TO THE LEFT & IS NO LONGER TRIMMED OUT IN COORDINATED FLIGHT AS IT WAS BEFORE AUTOPILOT WAS ENGAGED. RE-TRIMMED ACFT FOR COORDINATED FLIGHT, STRAIGHT + LEVEL. RE-ENGAGED YAW DAMP & AUTOPILOT, 2 OR 3 MINUTES LATER, ACFT STARTED TO YAW. NO QRH AVAILABLE. DISCONNECTED YAW DAMP AUTOPILOT & DIVERTED. ON DESCENT & APPROACH, SLOWED TO LESS THAN 250 KTS & ACFT STILL HAD A MILD YAW MOTION.