PARTIAL LOSS OF ACFT CTL WHEN THE PLT OF A DEP IFR C182 ENGAGES THE AUTOPLT AT 2000 FT WHICH THEN ROLLS THE ACFT 50 DEGS ON ITS LATERAL AXIS 3 NM SW OF SBA; CA.

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I HAD FILED AN IFR FLT PLAN FROM SANTA BARBARA TO PASO ROBLES. MY INTENTION WAS TO CLB ON TOP; CLR THE AREA; AND CANCEL. MY DEP WAS NORMAL; DEPARTED RWY 15R. CLBED TO 500 FT; TURNED TO 200 DEGS AND CONTINUED THE CLB TO 3000 FT. I WAS TO RECEIVE VECTORS TO GAVIOTA VOR. I WAS TOLD TO TURN TO 250 DEGS AND CONTINUE THE CLB. I HAD SET THE AUTOPLT AND THEN TURNED IT TO THE DESIRED COURSE. JUST AFTER I SET THE AUTOPLT IT ROLLED THE PLANE OVER TO A 45-50 DEG BANK. I TURNED OFF THE AUTOPLT AND CORRECTED THE ROLL. THIS TOOK A MIN OR SO. DURING THIS TIME; DEP CTL ASKED ME IF I HAD A PROB. I REPLIED THAT THE DIRECTIONAL GYROSCOPE HAD FAILED. SINCE THE EXACT HEADING AND ALTS ARE ONLY BEING RECALLED FROM MEMORY I EXPECT THAT THE GND TRACK AND ALT MAINTAINED BY ATC IS MUCH MORE ACCURATE THAN MY MEMORY. I WAS EXTREMELY BUSY TRYING TO FLY THE PLANE. CONTRIBUTING FACTORS? ENRTE FROM FHR TO SBA I LOST MY ENG DRIVEN VACUUM PUMP. I SWITCHED TO A STANDBY VACUUM PUMP AND CONTINUED THE FLT TO SBA. AT SBA THE VACUUM PUMP WAS REPLACED. WHEN I TOOK OFF; BOTH PUMPS WERE ON. ENRTE TO FHR I TRIED THE AUTOPLT IN THE SAME MANNER AS I USED DURING DEP (BUT IN VFR CONDITIONS). IT DID THE SAME THING; IT ROLLED THE PLANE OVER TO ABOUT 40 DEGS OF BANK. ON THE TRIP I CONTINUED TO USE THE AUTOPLT AND OBSERVED THAT IT WOULD HUNT FOR THE COURSE. IT WOULD WANDER BACK AND FORTH ABOUT 3-4 DEGS ON EITHER SIDE OF THE ENTERED COURSE. I AM INVESTIGATING TO DETERMINE THE CAUSE OF THE MALFUNCTION.