AN MD88 IN CLB EXPERIENCED A YAW OSCILLATION OF 1 CYCLE PER SECOND. NO CAUTION LIGHTS AND AUTOPLTS SWITCHED NO HELP. YAW DAMPER NOT SWITCHED OFF.

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CAPT'S LEG. AFTER TKOF AND THROUGH CLEAN-UP ALT; ACFT EXPERIENCED WHAT FELT LIKE WAKE TURB; ACCOMPANIED BY STEADY REQUIREMENT FOR AILERON DISPLACEMENT TO THE L; THEN R TO HOLD WING'S LEVEL. A LITTLE MORE THAN A UNIT OF TRIM TOOK OFF THE PRESSURE IN BOTH DIRECTIONS. AS ACFT ACCELERATED THROUGH CLEAN SPD; ALL FELT NORMAL; AND AILERON TRIM WAS RESET TO ZERO DEGS. AUTOPLT #1 WAS ENGAGED; AND ALL FELT NORMAL. APPROX 9000 FT. 250 KIAS A CONTINUOUS RUDDER OSCILLATION OF APPROX 1 CYCLE PER SECOND BEGAN ABRUPTLY; OF LIGHT; BUT STEADY INTENSITY. YAW DAMPER WAS VERIFIED ON. NO CAUTION LIGHTS WERE ILLUMINATED. AUTOPLT WAS DISCONNECTED TO NO EFFECT AND RE-ENGAGED. I CONTINUED CLB TO A TROUBLESHOOTING ALT OF 14000 FT AND 280 KIAS. FO INITIATED THE 'ACFT OSCILLATING OR DISPLACED IN YAW' CHKLIST. SWAPPED TO AUTOPLT #2 TO NO INITIAL EFFECT. THEN AFTER SEVERAL SECONDS OSCILLATION DISSIPATED AWAY. AUTOPLT #2 WAS ENGAGED; CLBOUT ACCELERATION WAS COMMENCED. APPROX 310 KIAS OSCILLATION ABRUPTLY RETURNED. I LEVELED AND SLOWED TOWARDS 250 KIAS. CHKLIST WAS CONTINUED; AND RUDDER HYD CTL WAS SELECTED TO MANUAL. OSCILLATION AGAIN DISSIPATED. CHKLIST WAS AGAIN SUSPENDED AS OSCILLATION CEASED. ALTHOUGH I CREDITED THE CEASING OF OSCILLATION TO THE REDUCTION IN SPD. I ELECTED TO KEEP RUDDER IN MANUAL FOR REMAINDER OF FLT. ELECTED TO RETURN TO ZZZ FOR LNDG. NIGHT VFR LNDG WAS MADE WITH NO FURTHER PROBS. DID NOT DECLARE AN EMER. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE CAUSE OF THE RUDDER OSCILLATION IS UNKNOWN AS MAINT HAS NOT RPTED THE ACTION TAKEN OR THE MALFUNCTIONING COMPONENT. THE RPTR SAID FLT MANUAL PROCS WILL NOT ALLOW DISCONNECTING THE YAW DAMPER FOR TROUBLESHOOTING.