B737 UNCOMMANDED ROLL AT FL280.

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WE DEPARTED LAX FOR SFO ON A CLR EVENING. APPROX 50 PAX; 5 CREW MEMBERS ON BOARD. WE WERE FLYING DIRECT TO RZS AT FL310 AT ABOUT .77 MACH IN CLR AND MOSTLY SMOOTH AIR. WIND WAS FROM THE NW; LNAV; VNAV AND SYS B AUTOPLT WERE ENGAGED; BOTH FLT DIRECTORS WERE ON. YAW DAMPER WAS ENGAGED. 2 ACFT WERE AHEAD AT FL310; 10 MI AND 25 MI. WE ASKED FOR FL280. WE BEGAN TO PICK UP VERY LIGHT CHOP AT FL310. ATC CLRED US TO DSND AND I RESET THE ALT WINDOW; THE FO RESET THE CABIN ALT. AS I LEANED FORWARD TO ENTER FL280 IN THE FMC; THE ACFT ABRUPTLY ROLLED HARD TO THE L. NO FLT CTL INPUT HAD BEEN MADE BY EITHER PLT. WE BOTH GRABBED THE YOKE; DISCONNECTED THE AUTOPLT AND STEPPED ON THE R RUDDER WHICH IMMEDIATELY BROUGHT THE PLANE UPRIGHT. I ESTIMATE A MAX OF 20-30 DEGS OF BANK WAS ENCOUNTERED BUT THE ROLL RATE WAS VERY FAST. NO PAX WERE HURT; 1 FLT ATTENDANT WAS THROWN TO THE FLOOR BUT WAS NOT HURT. WE DECLARED AN EMER AND ASKED TO RETURN TO LAX. ATC COMPLIED AND WE STATED THE NATURE OF THE EMER WAS A 'FLT CTL PROB.' WE ELECTED TO DISCONNECT THE YAW DAMPER MANUALLY IN CASE THAT WAS THE CAUSE OF THE PROB. THE FLT CONTINUED UNEVENTFULLY TO LAX. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THE ACFT INVOLVED IA B737-500. AFTER THE EMER LNDG IN LAX; A FLT SAFETY INVESTIGATION TOOK PLACE. THE FINDINGS OF THE INVESTIGATION INDICATED A MALFUNCTION IN THE AUTOPLT AFFECTING THE ROLL CHANNEL. THERE WAS NO RUDDER OR RUDDER VALVE INVOLVEMENT.