WIDEBODY FREIGHT ACFT EXHIBITED AUTOPILOT MALFUNCTIONS DURING FLT. DURING APCH; FLT CREW DISCONNECTED THE AUTOPILOT WHEN IT BECAME APPARENT THAT IT WOULD OVERSHOOT THE LEVEL-OFF ALT.

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WE WERE ON THE FINAL PORTION OF A 9+35 FLT. ENRTE; WE HAD A COUPLE OF HARD JOLTS ON THE AUTOPLT AT ALT. WE FELT SOMETHING WAS GOING ON WITHIN THE AUTOPLT SYS THAT WE SHOULD WATCH. WE EVENTUALLY CHANGED TO THE #2 SYS; WHICH SEEMED TO RESPOND NORMALLY THE REST OF THE WAY ACROSS THE PACIFIC. DURING DSCNT; WE AGAIN GOT SEVERAL JOLTS ON THE AUTOPLT BUT IT DID NOT DISCONNECT. I SWITCHED BACK AND FORTH FROM #1 TO #2 AUTOPLT ATTEMPTING TO SEE WHICH SYS WOULD BE OUR BEST OPTION FOR THE APCH. AT ABOUT 12000 FT WE GOT THE FIRST OF SEVERAL STABILIZER OUT OF TRIM ALERTS. WE LOOKED UP THE ALERT AND KNEW THAT WHEN DISCONNECTING THE AUTOPLT THE ACFT MAY BE OUT OF TRIM. WE DISCONNECTED THE AUTOPLT ONCE TO SEE IF THE TRIM WAS GOING TO BE OK. IT SEEMED TO BE REASONABLY TRIMMED; AND WAS RESPONDING NORMALLY TO MANUAL TRIM SO THE AUTOPLT WAS AGAIN CONNECTED. WE WERE IN AND OUT OF THE CLOUDS AFTER A LONG FLT ACROSS THE PACIFIC; SO I FELT THE BEST OPTION WAS TO USE THE AUTOPLT IF WE FELT COMFORTABLE THAT ONE OF THE SYS WAS GOING TO FUNCTION ADEQUATELY. WE WERE GIVEN A DSCNT TO 4000 FT. AS WE APCHED THAT ALT; THE AUTOPLT SEEMED TO BE PERFORMING OK; BUT AS WE APCHED THE LEVELOFF POINT; IT BECAME OBVIOUS THAT THE ACFT WAS NOT GOING TO LEVEL AS REQUIRED. AGAIN WITH A STABILIZER OUT OF TRIM ALERT WE DISCONNECTED THE AUTOPLT TO LEVEL OFF; BUT WITH THE TRIM STILL A LITTLE OUT OF NEUTRAL THE ACFT DSNDED ABOUT 200-250 FT BELOW OUR LEVELOFF ALT. WE IMMEDIATELY CLBED BACK TO ALT AND LEVELED OFF. AT THIS POINT WE DECIDED TO JUST FLY THE APCH WITH THE AUTOPLT OFF. APCH CTL NEITHER ASKED NOR NOTED ANY ALTDEV TO US. THE FO WAS VERY HELPFUL DURING THIS ENTIRE PROCESS BOTH CALLING OUT DEVS AND BACKING UP ON HDGS AND ALTS DURING THE AUTOPLT OFF OP OF THE ACFT. THE ACFT WAS WRITTEN UP. THIS INCIDENT ILLUSTRATES TO ME THE GRAY AREA THAT EXISTS; TRANSITIONING BTWN WANTING TO STAY WITH THE AUTOMATION; WHICH YOU KNOW UNDER NORMAL CIRCUMSTANCES FLIES VERY PRECISE PARAMETERS; AND THE DECISION TO ELIMINATE THE AUTOMATION AND FLY THE ACFT.