WDB AUTOPLT FAILED TO TRACK ILS ON 2 CONSECUTIVE APCHS.

Narrative

WE INITIALLY WERE IN A HOLDING PATTERN AT WILLOW INTXN AND THEN WERE RADAR-VECTORED FOR AN APCH ILS RWY 26L AT LONDON GATWICK ARPT. SINCE THE WX WAS IMC WE ELECTED TO EXECUTE A MONITORED APCH. AUTOPLT 'A'; AUTOPLT 'B' AND THE 'LAND' SYS WERE ENGAGED. INITIALLY THE LOC AND THE GS SEEMED TO BE CAPTURED. SHORTLY THEREAFTER; HOWEVER; WE NOTICED THAT THE AIRPLANE WAS DRIFTING TO THE R OF THE LOC. I TOLD THE FO TO DISCONNECT THE AUTOPLTS; TURN L AND REINTERCEPT THE LOC. WE STILL HAD A R OF COURSE INDICATION; SO WE EXECUTED A MISSED APCH. THIS WAS DONE WELL ABOVE MINIMUMS. WE EXECUTED THE AFTER TKOF AND APCH CHKLISTS. WE RETUNED AND REIDENTED THE ILS FREQ AND I DECIDED TO EXECUTE ANOTHER COUPLED APCH; THIS TIME USING AUTOPLT 'A' ONLY SINCE THIS WAS THE HIGHER GRADE AUTOPLT. AGAIN; THE LOC SEEMED TO BE INTERCEPTED INITIALLY. WE THEN NOTICED THAT THE ACFT DRIFTED TO THE R OF COURSE AND THIS WAS CONFIRMED BY APCH CTL; WHO SUGGESTED A HDG OF 265 DEGS AND MAINTAIN A 3:1 GS RATIO. HAVING GND CONTACT WE SAW THAT WE WERE NOT LINED UP WITH THE RWY AND EXECUTED A MISSED APCH. AGAIN; THIS WAS WELL ABOVE MINIMUMS. SUSPECTING AUTOPLT TRACKING MALFUNCTION; I DECIDED TO EXECUTE A THIRD AND FINAL APCH; THIS TIME MANUAL. WE AGAIN RETUNED AND REIDENTED THE ILS FREQ. THIS WAS TO BE OUR FINAL APCH BEFORE PROCEEDING TO OUR ALTERNATE OF PRESTWICK. THIS TIME THE LOC WAS CAPTURED NORMALLY; THE APCH COMPLETED AND WE LANDED WITHOUT FURTHER INCIDENT. WE LANDED WITH 65000 LBS OF FUEL. I WROTE UP THE INCIDENT IN OUR MAINT LOGBOOK.