FLC WAS AT FL350 AT CRUISE CLRED DIRECT TO A DISTANT VOR AFTER BEING GIVEN A MOMENTARY 10 DEG VECTOR CORRECTION AND MADE AN UNEXPECTED 65 DEG TURN TO THE R FOLLOWED BY A TURN BACK TO THE L. IMMEDIATE CORRECTIVE ACTION WAS REQUIRED BY THE ALERT CTLR TO PRESERVE THE SPACING WITH THE TFC IN TRAIL.

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WE WERE ON A FLT FROM CVG TO SFO; NAV WAS OMEGA; BACKED WITH VOR. WE WERE ON A DIRECT TO COALDALE. CTR REQUESTED A 10 DEG TURN; THEN AGAIN CLRED US TO GO DIRECT TO COALDALE. THE COPLT WAS FLYING; AND I CHANGED MY VOR TO THE NEXT STATION; MODESTO. I WAS ENGAGED IN CONVERSATION WITH THE SO; AND TURNED AROUND NOT OBSERVING THE FLT PATH. THE COPLT MADE A TURN TO THE R FOLLOWED BY A TURN TO THE L. WHILE WE WERE IN THE L TURN (APPROX 270 DEGS) CTR CALLED AND TOLD US TO MAKE AN IMMEDIATE L TURN TO 210 DEGS. CTR ALSO TOLD ANOTHER FLT BEHIND US TO MAKE AN IMMEDIATE R TURN. WE OBSERVED THE TCASII AND A TARGET WAS TO OUR R REAR NEAR THE 10 MI MARK. THE FO HAD MADE THE TURN TO CORRECT BACK ON COURSE; NOT REALIZING I HAD CHANGED THE VOR STATION. WHEN I OBSERVED THE FLT PATH WE WERE BACK ON COURSE. CTR CALLED US AGAIN AND ASKED US WHY WE MADE SUCH AN ABRUPT CHANGE IN HDG (APPROX 65 DEGS). I TOLD THE CTR WE WERE CORRECTING ON COURSE. CTR STATED IN THE FUTURE IF WE MAKE THAT BIG A CHANGE IN HDG TO NOTIFY THEM FIRST. CONTRIBUTING FACTORS: LONG FLT TIME; NEAR THE END OF THE FLT. COMPLACENCY WITH A NAV SYS THAT WAS FLYING DIRECT; AND DISTR IN THE COCKPIT WITH CONVERSATION. FAILURE TO NOTIFY ATC OF THE TURN.