B757-200 CREW HAD UNCOMMANDED RUDDER OSCILLATIONS AFTER DEP.

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CAPT WAS PF. AFTER FLAP RETRACTION; 3000 TO 4000 FT MSL; 250 KIAS; ATC INSTRUCTED R TURN HDG 280 DEGS. UPON ENTERING TURN; ACFT BEGAN TO OSCILLATE/SHIMMY 2 TIMES PER SECOND; RATHER VIOLENTLY; SIMILAR TO WAKE ENCOUNTER; DECREASED SLIGHTLY; THEN INCREASED AGAIN AND CONTINUED TO FISHTAIL PER AFT FLT ATTENDANT FOR ABOUT 30 SECONDS. ACFT RETURNED TO NORMAL FLT. DECIDED TO MAKE PRECAUTIONARY RETURN TO DFW. ON VERY SHORT FINAL; ACFT BEGAN SIMILAR FISHTAILING; BUT NOT AS SEVERE AS FIRST ENCOUNTER. THIS WAS BELOW 50 FT AGL RIGHT BEFORE TOUCHDOWN. HAD REVIEWED OVERWT LNDG IN QRH BEFORE LNDG. LANDED FLAPS 25 DEGS; 217500 LBS WT; VERY SOFT TOUCHDOWN AT ABOUT 50 FPM DSCNT. RETURNED TO GATE AND BRIEFED MAINT ON OSCILLATION AND OVERWT LNDG. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE PLT RPTS THEY DID NOT HAVE TIME TO DEACTIVATE THE YAW DAMPER SYS. THEY DO NOT KNOW WHETHER THIS HAD ANYTHING TO DO WITH THE PROB OR NOT. THE RPTR INSPECTED THE ACFT MAINT LOGS. ALL 3 RUDDER PWR CTL UNITS WERE REPLACED. THE ACFT WAS THEN TEST FLOWN; WITH NO REPEAT OF THE PROB. THE B757 HAS NO ABNORMAL OR EMER PROC FOR UNCOMMANDED RUDDER ACTUATION THAT SOME OTHER ACFT HAVE.