B737 CREW HAD UNCOMMANDED CTL INPUT AND UNRESPONSIVE FLT CTLS ON FINAL APCH.

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UNCOMMANDED ROLL/YAW. I PICKED UP ACFT (A B737-300) AT ZZZ1 FOR THE FIRST LEG OF A 2 LEG DAY TO ZZZ. THE CAPT THAT BROUGHT THE AIRPLANE IN HAD NO COMPLAINTS NOR WERE THERE ANY LOG WRITE-UPS. WE HAD 1 1/2 HRS GND HOLD AT ZZZ1 AT THE GATE DUE TO TFC AND WX AT ZZZ2. FO FLEW THE LEG FROM ZZZ1 TO ZZZ2. EVERYTHING WENT NORMAL AND ALL APPEARED TO BE OK WITH THE ACFT. I DID NOTICE AND MENTIONED THAT MY LOC DID NOT PICK UP A SIGNAL UNTIL 3 OR 4 MINS AFTER HIS DID. ON THE GND AT ZZZ2; INSTEAD OF CHANGING ACFT; WE WERE TOLD TO KEEP THIS ACFT FOR THE LEG TO ZZZ. I FLEW THE LEG FROM ZZZ2 TO ZZZ3 WITH THE ACFT FLYING NORMAL DURING TKOF; CLBOUT AND LEVELOFF. IT WAS A NORMAL LEG FLOWN AT THE FLT PLANNED ALT OF FL310. DSCNT WAS NORMAL WITH NO ERRATIC PERFORMANCE. TFC WAS LIGHT AND WE WERE CLRED FOR THE ILS. THERE WERE NO ACFT IN FRONT OF US WHILE WE WERE ON THE APCH AND THE WINDS WERE LIGHT TO CALM. THIS BEING MY FIRST TIME INTO ZZZ; I PURPOSELY SLOWED DOWN AND STARTED CONFIGURING THE ACFT A LITTLE EARLY. I CALLED FOR GEAR DOWN AND FLAPS 15 DEGS AT THE 11 DME POINT FROM THE RWY. I WAS ON THE GS HAND FLYING THE ACFT WITH THE AUTOTHROTTLES ENGAGED. BUGGING DOWN TO 150 KTS WITH FLAPS 15 DEGS; I CALLED FOR FLAPS 25 DEGS AND 30 DEGS AND BUGGED DOWN TO BUG SPD +5 KTS. THE AUTOTHROTTLES RETARDED THE THROTTLES TO IDLE (AT 8 DME FROM FIELD). WHEN THE SPD APCHED BUG THE THROTTLES ADVANCED TO MAINTAIN THE DIALED IN SPD WHICH WAS AT 134 KTS. AT THIS POINT THE ACFT ROLLED R THEN L AT 10 DEGS; 2-3 TIMES AND THERE SEEMED TO BE SOME YAW FIRST R THEN L. I DISENGAGED THE AUTOTHROTTLES AND SAID TO THE FO; 'I GUESS THAT WAS ASYMMETRICAL THRUST (SPOOL UP).' WHAT WAS SO NOTICEABLE ABOUT THE EVENT WAS THAT WHEN I TRIED TO COUNTERACT THE CONDITIONS THE CTLS WERE VERY SOFT TO ALMOST NON EXISTENT. I CONTINUED ON THE APCH WITH THE RWY IN SIGHT AND AFTER SAY 10-20 SECONDS WITH SPD AND DSCNT STABILIZED; THE ACFT STARTED THE UNCOMMANDED ROLLING MOTION AGAIN; AGAIN THE FLT CTLS DID NOT INITIALLY STOP NOR CORRECT THE ACTION. AGAIN THE CTLS DID NOT APPEAR TO BE RESPONDING TO INPUTS NORMALLY. IT TOOK EXAGGERATED L AILERON TO STOP THE ROLLS TO THE R AND VICE VERSA; WITH THE RUDDER FEELING SOFT AND MUSHY AND NOT RESPONDING TO INPUTS. ONCE AGAIN I REGAINED CTL. (AT THE ONSET OF THE SECOND SERIES OF ROLLS I CALLED FOR YAW DAMPER OFF AND AT THE SAME TIME REACHED UP AND TURNED IT OFF.) AT THIS POINT I ASKED WHAT WERE THE WINDS FROM PAGE 2 OF THE PROGRESS HDG IN THE FMC. HE SAID 'CALM.' ALL APPROPRIATE CHKLISTS HAD BEEN CALLED FOR AND COMPLETED I MIGHT ADD. FOR A SHORT TIME -- SAY 5-10 SECONDS -- CTL INPUTS APPEARED TO RETURN TO NORMAL; AND THEN FROM SAY 500-800 FT OUT THEY APPEARED TO BE GRADUALLY JUST FADING OUT WITH THE ACFT CONTINUING THE ROLL ACTION WITH VERY LITTLE RUDDER RESPONSE. IN ANY NORMAL ACFT I WOULD HAVE GONE AROUND AT THIS POINT; BUT AT THIS POINT I DID NOT FEEL THE AIRPLANE WAS AIRWORTHY. I DECIDED I HAD TO LAND EVEN IF IT MEANT A HARD LNDG DUE TO LACK OF NORMAL CTL INPUTS. I HAD MARGINAL RUDDER AND AILERON CTL AT TOUCHDOWN. I USED NOSE-UP TRIM AT FLARE AND IT GAVE ME A NORMAL LNDG. FROM THE 500 FT POINT ON DOWN; I CONFIRMED SYMMETRICAL THRUST; NORMAL SINK AND REALIZED THAT I WOULD HAVE TO ACCEPT DRIFT AT TOUCHDOWN RATHER THAN HAVE AN EXAGGERATED AILERON INPUT IN AND HAVE THE CTLS RETURN TO NORMAL AND HIT AN ENG OR WINGTIP. ROLLOUT AND REVERSE APPEARED TO BE NORMAL. TAXI IN WAS NORMAL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE CREW WAS ON THE SECOND LEG OF THE DAY WITH THE ACFT. ON APCH; WITH FLAPS 15 DEGS AND APPROX 150 KTS; THE ACFT BEGAN AN UNCOMMANDED YAW AND ROLL. AT THIS TIME THE ROLLING MOTION WAS ABOUT 10 DEGS FROM SIDE-TO-SIDE AND BECOMING DIVERGENT. THE CREW DISCONNECTED THE AUTOPLT AND AUTOTHROTTLES; NO CHANGE. THE CAPT THEN VERIFIED THAT THE ENGS WERE NOT SURGING BY OBSERVING NO CHANGE IN N1 OR FUEL FLOW. NEXT; THE YAW DAMPER WAS TURNED OFF; AGAIN NO CHANGE. HYD SYS 'A' AND 'B' WERE VERIFIED AS NORMAL. ALL THIS TIME THE CAPT WAS ATTEMPTING TO CTL THE ROLL WITH CTL YOKE INPUT. USING THE TECHNIQUE HE WAS TAUGHT TO CTL DUTCH ROLLS; HE INPUTTED SHARP AILERON AND SPOILER INPUTS. CTL YOKE ANGLES OF AT LEAST 45 DEGS WERE USED. AT NO TIME DID THE CAPT FEEL ANY UNWANTED MOVEMENT IN THE RUDDER PEDALS. THE CREW DID NOT ATTEMPT TO TURN ON STANDBY RUDDER BECAUSE OF THE LOW ACFT ALT. THE FO BECAME CONCERNED THAT IF ALLOWED TO CONTINUE FOR SEVERAL MORE CYCLES; THE ACFT WOULD GO ON ITS BACK. THE CAPT FELT THE LATERAL AND RUDDER CTL WERE SOFT AND DID NOT PROVIDE NORMAL RESPONSE. THE CAPT DEBRIEFED THE ACR MAINT DEPT; AND HAD AN EXTENSIVE DISCUSSION WITH ACR FLT TEST DEPT. THE ACFT WAS GIVEN AN EXTENSIVE FLT TEST AND FERRIED BACK TO THE ACR MAIN MAINT BASE. THE ONLY THING THAT WAS FOUND WAS A LOOSE FLAP FAIRING CANOE ON THE L WING. THE CAPT DOES NOT FEEL THAT THIS COULD HAVE CAUSED THE PROBS HE EXPERIENCED.