AN A320 CAPT DESCRIBES AN AIRFRAME VIBRATION AND YAW STARTING AT ABOUT FL280. VIBRATION STOPPED WHEN MACH/AIRSPEED SET AT .78.

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BETWEEN FL280 AND FL290; ACFT STARTED VIBRATING OR SHAKING IN YAW. WE WERE FLYING 300 KIAS ATC ASSIGNED SPEED (ABOUT .78M). THE VIBRATION CHECKLIST WAS ACCOMPLISHED; AND THE VIBRATION WENT AWAY WITH RUDDER INPUT IN EITHER DIRECTION. HIGHER ALTITUDES AND LOWER SPEEDS BOTH MADE THE VIBRATION LESS OR GO AWAY COMPLETELY. AS A TEST; ONCE AT CRUISE; FL360; WE ACCELERATED TO .80M. VIBRATION THEN WAS COMING IN WAVES; SLIGHT THEN BUILDING IN INTENSITY TO A POINT; THEN OFF. A SLOW BUILD TO A MAX INTENSITY; THEN STOP. OVER AND OVER. WE SLOWED TO .78 AND HAD NO PROBLEMS WHATSOEVER. CONTINUED TO DEST WHERE WE HAD AGREED WITH MAINT AND CHIEF PLT THAT IT SHOULD BE PARKED UNTIL FIXED. THIS IS THE SAME AIRCRAFT THAT I HAD THE SAME PROBLEM WITH SEVEN DAYS EARLIER. IT WAS ALSO WRITTEN UP BY ANOTHER CREW THE DAY BEFORE.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER FOLLOWED UP ON THE COMPLETED MAINTENANCE OF THIS ACFT. HE DISCOVERED THAT THE 'GREEN' AND 'BLUE' RUDDER ACTUATORS WERE OUT OF TOLERANCE BY 12 MM. ALSO THE UPPER AND LOWER RUDDER SERVOS AND ROD ENDS WERE REPLACED. HE REPEATED THAT THE ACFT ACTIONS DURING THIS EVENT WERE VERY PRONOUNCED.