A B737-300 ENTERS AN UNCOMMANDED ROLL AT FL310. FLC REGAIN CTL OF ACFT BY TURNING OFF 'A' AUTOPLT; 'B' YAW DAMPER; AND 'B' FLT CTL.

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WHILE IN CRUISE AT FL310 ACFT YAWED L WITH A ROLL; THEN R WITH A ROLL; THEN BACK L. AT THIS POINT THE AUTOPLT WAS DISCONNECTED ALONG WITH THE YAW DAMPER. THE 'B' SYS FLT CTL SWITCH WAS PLACED TO STANDBY RUDDER. THE UPSET WAS STOPPED. THE 'A' AUTOPLT WAS RE-ENGAGED ALONG WITH 'B' SYS FLT CTL. IN THE MIDDLE OF R TURN YAW BEGAN AGAIN. 'B' SYS FLT CTL WAS SWITCHED TO STANDBY RUDDER. FLT LANDED UNEVENTFULLY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THE UNCOMMANDED ROLL BEGAN WITH A SWIFT PULSE OR KICK. IT ROLLED UP TO A 15-20 DEG BANK BEFORE THE BANK WAS ARRESTED. THE FLC STOPPED THE BANK BY TURNING OFF THE 'A' AUTOPLT; 'B' YAW DAMPER; AND 'B' FLT CTL WAS SWITCHED TO STANDBY RUDDER. RPTR ESTIMATES IT TOOK 5 SECONDS FOR THE ROLL TO STOP. THE ROLL DID NOT STOP UNTIL ALL 3 CTL SWITCHES WERE PLACED OFF. ACFT WAS FLOWN FOR A WHILE AND THE 'A' AUTOPLT WAS SWITCHED ON ALONG WITH 'B' FLT CTL. IN THE MIDDLE OF A TURN; YAW BEGAN AGAIN. 'B' SYS FLT CTL WAS SWITCHED TO STANDBY RUDDER AND THE YAW STOPPED. THE ACFT WAS A B737-300. RPTR STATED THAT HE HAS HAD TRAINING IN THE SIMULATOR REGARDING UNCOMMANDED ROLL AND HAD THE PROC IN MIND; TURNING OFF THE 3 SWITCHES; SHOULD AN UNCOMMANDED ROLL BE ENCOUNTERED. THE TRAINING WAS VERY HELPFUL TO HIM. MANY OF THE PAX NOTICED THE ABRUPT ROLLING TURN AT ALT AND COMMENTED ON LNDG THAT THEY KNEW SOMETHING WAS WRONG WITH THE CTLS. APCH AND LNDG WERE NORMAL AFTER ALL THIS. THERE HAS BEEN NO RELEASE OF ANY FURTHER INFO FROM THE COMPANY AS TO ANY MECHANICAL FINDINGS OF WHAT CAUSED THE UNEXPECTED ROLL.