AUTOPLT ABRUPTLY ROLLS TO INTERCEPT ON COURSE.

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FL330 INITIAL PROB WAS MALFUNCTIONING YAW DAMPER. PROB ISOLATED AND DAMPER TURNED OFF. TURB IN FLT WITHOUT YAW DAMPER ON CAUSED SOME RIDE PROBS. AT SOME POINT AUTOPLT WAS TURNED OFF TO HAND FLY. UPON RE-ENGAGEMENT AND SELECTION OF 'LNAV' BACK ON COURSE; ACFT ENTERED RAPID R BANK. AUTOPLT DISCONNECTED. ACFT CTL MAINTAINED. AGAIN B AUTOPLT RESELECTED. LNAV AGAIN RESELECTED WITH ACFT OFF L NAV COURSE ONE BAR WIDTH. ACFT AGAIN ENTERED RAPID R BANK. AUTOPLT AGAIN DISCONNECTED. MANUALLY USING CTL WHEEL STEERING ESTABLISHED ON COURSE. AUTOPLT 'B' SYS RESELECTED AND NO PROBS ON REMAINDER OF FLT. CREW DISCUSSED DIVERT INTO OMAHA; BUT AS NO OTHER PROB; CONTINUED ON. CREW DID NOT UNDERSTAND REASON FOR RAPID R ROLL; BUT MONITORED CAREFULLY. IN RETROSPECT; SHOULD HAVE SELECTED 'A' AUTOPLT; WHICH MIGHT HAVE PREVENTED SECOND R ROLL. OUR INITIAL THOUGHTS WERE OF PITTSBURG PROB AND CAUSED SOME CONCERN. TOOK ABOUT 1 HR TO NARROW DOWN TO AUTOPLT AS YAW DAMPER HAD BEEN ELIMINATED. BUT WE STILL HAD DOUBT AS RATE OF ROLL WAS INAPPROPRIATE FOR FLT SIT. AT NO TIME DID WE DEV FROM ASSIGNED ALT OR RTE; BUT IT WAS AN INTERESTING TIME. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THEY WERE FLYING A B737-300 FROM SFO TO ORD. THE RPTR HAD 23 YRS IN THE MIL MUCH OF THAT TIME AS AN INSTRUCTOR IN AEROBATICS AND UNUSUAL ATTITUDES. THE VISIBILITY IN FLT WAS ABOUT 1 MI AND THE HORIZON WAS NOT VISIBLE. AT THE TIME OF THE OCCURRENCE THE FLC WAS FOCUSED ON FLYING THE AIRPLANE WITH THE EXISTING TURB AND YAW DAMPER OFF THE RIDE WAS NOT COMFORTABLE. THE AIRPLANE WAS WELL WITHIN ITS PERFORMANCE CAPABILITY AND CG LIMITS. WHEN SELECTING AUTOPLT ON AND LNAV MODE THE AILERON CTL WHEEL ABRUPTLY TURNED TO PRODUCE A RAPID R BANK AND THE AUTOPLT DISCONNECTED. AT NO OTHER TIME IN HIS 4500 HRS IN THE B737 HAD HE EXPERIENCED AN UPSET OR A PROB LIKE THIS.