FLC OF A DC8 FAILED TO FOLLOW ATC TRACK INSTRUCTIONS DURING DSCNT RESULTING IN ATC INTERVENING AND ISSUING AN IMMEDIATE COURSE CHANGE TO AVOID A CONFLICT WITH ANOTHER ACFT.

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CAPT HAD 3 MAX DUTY DAYS WITH MINIMUM REST (ALL NIGHT FLYING). DAY STARTED IN DFW AT XX30L WITH HUB STOP AT HUF. WAS TOLD BY SCHEDULER HE WAS TO BE RELEASED TO GO HOME AT HUF. ARRIVED HUF AND FOUND OUT HE WAS TO OPERATE TO SFO. WHOLE CREW WAS VERY TIRED. ON DSCNT (CAPT FLYING) WITH AUTOPLT COUPLED TO GPS #1 ZOA CLRED US DIRECT TO CEDES INTXN TO DSND TO FL240. WE WERE ABOUT OVER MDO VOR AND I HAD THE MDO 245 DEG RADIAL DIALED IN. STARTED DSCNT AND STARTED PUTTING CEDES IN THE GPS. HSI SHOWED US R OF COURSE; AUTOPLT COUPLED TO GPS; ACFT STARTS A L TURN TO INTERCEPT. (IN FACT; STRAIGHT AHEAD WAS PRETTY MUCH ON COURSE; WE WERE ALMOST ON TOP OF MDO VOR SO OF COURSE THE NEEDLE WAS FULL SCALE L.) APCHING FL330 ZOA ISSUED AN IMMEDIATE R TURN TO 270 DEGS (ACFT HDG WAS ABOUT 235 DEGS). AS FAR AS I KNOW NO VIOLATION WAS ISSUED BY ZOA BECAUSE NOTHING WAS FURTHER SAID ABOUT IT. LOOKING BACK; FATIGUE WAS A LARGE PLAYER; PLUS COCKPIT WORKLOAD -- RUNNING CHKLISTS; ETC; LED TO DISTR ON MY PART. I SHOULD HAVE UNCOUPLED NAV FROM AUTOPLT BECAUSE WE WERE ONLY 1/2 - 1 MI R OF COURSE. THE AUTOPLT CRANKED IN A LARGE INTERCEPT BECAUSE OF THE FULL L DEFLECTION SINCE WE WERE SO CLOSE TO THE VOR. FACTORS WERE 3 MAX DAYS IN A ROW -- ALL AT NIGHT AND THE TIME OF MORNING (JUST GETTING LIGHT). I WAS SEEING THE RIGHT INDICATIONS; BUT THEY WERE NOT REGISTERING IN THE BRAIN LIKE THEY SHOULD; DUE TO FATIGUE. LESSON LEARNED WAS TO UNCOUPLE AUTOPLT FROM NAV UNIT AND PUT IT ON HDG MODE AND TO TRY TO BE MORE ALERT.