B757 REQUIRED EXCESSIVE RUDDER TRIM DURING CRUISE. THE ACFT ALSO EXHIBITED MINOR PITCH PULSING ON FINAL APCH.

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THIS ACFT HAD 1 YAW DAMPER PLACARDED INOP AND ALSO REQUIRED A LOT OF RUDDER TRIM DURING THE FLT; FOR EXAMPLE 4 UNITS AT 300 KTS (THIS WAS ENTERED AS A SEPARATE WRITE-UP). ALTHOUGH WE CHKED THE AILERON TRIM BEFORE ENG START; WE DID NOT CHK AILERON TRIM IN CRUISE BY DISCONNECTING THE AUTOPLT. LATER IN THE FLT WE EXPERIENCED A SEPARATE AND UNRELATED PROB; PITCH PULSES ON SHORT FINAL. ON A STABLE APCH INTO BOS (ILS RWY 4R) WITH BRAG; A 20 KT QUARTERING HEADWIND; BEHIND A HVY AIRBUS; WE NOTICED A RHYTHMIC PULSING IN THE YOKE THAT STARTED AT APPROX 300 FT AND CONTINUED INTO THE FLARE (THIS WAS ENTERED AS A SEPARATE WRITE-UP). I HAD SOME CONCERNS ABOUT ACFT CTL; BECAUSE THE YOKE PULSE WAS A VERY SMALL PITCH UP; FOLLOWED BY A VERY SMALL PITCH DOWN. IT FELT LIKE SOME KIND OF AN ACFT INPUT TO THE YOKE. HOWEVER; I DO NOT RULE OUT THE POSSIBILITY OF SOME KIND OF UNUSUAL WINDSHEAR OR VORTEX EFFECT FROM THE TFC AHEAD. THE AUTOPLTS AND AUTOTHROTTLES WERE OFF AND THERE WERE NO TRIM INPUTS; ICE BUILD-UP OR BIRD STRIKES. DUE TO THIS PITCH ANOMALY; BELOW 300 FT; I GOT HIGH ON THE GS. ALTHOUGH I LANDED A LITTLE LONG (APPROX 2500 FT PAST THRESHOLD); I ELECTED TO CONTINUE THE LNDG AS THE SAFEST COURSE OF ACTION. I MADE SMOOTH FLARE INPUTS; SENSING THAT THE LITTLE UP AND DOWN PITCH PULSES WOULD CANCEL EACH OTHER OUT. THAT WAS THE CASE AND THE FLARE AND LNDG WERE SMOOTH. I DISCUSSED THE RUDDER TRIM AND PITCH EVENT IN DETAIL WITH VARIOUS MECHS; MADE 2 SEPARATE ENTRIES AND EVEN SUGGESTED THAT THEY PULL THE FLT DATA RECORDER. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT MAINT REPLACED THE YAW DAMPER SERVO THE FOLLOWING DAY; BUT COULD NOT DUPLICATE AN ELEVATOR ABNORMALITY. SUBSEQUENT FLTS DID NOT RESULT IN ANY PITCH ANOMALIES.