A B767 EXECUTED A GAR ON A CAT III APCH AFTER THE ACFT EQUIPMENT CAPABILITY DOWNGRADED TO CAT 1. THE FLT DIVERTED TO AN ALTERNATE.

Narrative

RPTED WX CONDITIONS REQUIRED A CAT 3 APCH AND AUTO LAND. ON CTR AUTOPLT INTERCEPTED LOC AND GLIDEPATH TO ZZZZ RWY 17L. AT APPROX 2800 FT ABOVE THE RWY ACFT CTR AUTOPLT AGGRESSIVELY PITCHED THE ACFT NOSE DOWN THEN UP AT WHICH TIME THE AUTOPLT DISCONNECTED WITH WARNING. I ENGAGED THE R AUTOPLT AND THE LOC AND GS INTERCEPTED INDICATED CAPTURED FOR A MOMENT. THE ACFT PITCHED DOWN MORE AGGRESSIVELY THAN THE FIRST TIME THEN UP VERY RAPIDLY. AT THE SAME TIME THE FO STATED HIS LOC AND GS CAPTURE MODES ON HIS ADI/PDI BOTH HAD RED LINES THROUGH THEM. I ENGAGED THE L AUTOPLT. THE LOC AND GS INDICATIONS WERE CAPTURED; STILL IN THE SLOT ON SPD AS I CONTINUED TO FLY AND CONFIGURE ACFT THROUGH THE MALFUNCTIONS. THE L AUTOPLT SEEMED JUST FINE. MY LOC AND GS INDICATIONS REMAINED NORMAL; CAPTURED WHEN CONNECTED TO L AUTOPLT. THE FO QUESTIONED TWR REGARDING RWY 17L VISIBILITY; WE COULD NOT SEE THE RWY. TWR CTLR RESPONDED WITH RWY VISIBILITY THAT REQUIRED AN ACFT CAT 3 AUTO LAND OP. AT 1700 FT AGL; I REENGAGED CTR AND R AUTOPLT. AT 1400 FT AGL; ACFT COMPLETED ITS OWN TEST AND INDICATED LAND 3. THE FO'S FLT INST INDICATIONS WERE NOW NORMAL LOC AND GS CAPTURED. FOR A MOMENT; EVERYTHING WAS COPASETIC. AS WE DSNDED THROUGH 700 FT AGL THE ACFT ASA DOWNGRADED EQUIVALENT TO CAT 1 STATUS; SO I COMPLETED A NORMAL MISSED APCH; GAR PROC. EVEN DIRECTLY OVERHEAD; I DID NOT VISUALLY ACQUIRE ANY PORTION OF RWY 17L. ON TOP OF THE FOG BANK BLANKETING THE RWY; IT WAS A BEAUTIFUL DAY IN ZZZZ. THAT'S ALL THE HALF ENGLISH SPEAKING PAX WOULD REMEMBER. ZZZZ OPS TOLD OUR FLT ATTENDANTS AND CUSTOMERS THAT THERE WAS NO REASON FOR US TO GO AROUND; THE VISIBILITY OVER THEIR DESK TOPS WAS PERFECT. OUR CURRENT ACFT AND RWY STATUS; COUPLED WITH ACFT ARRIVING ZZZZ FROM ALL DIRECTIONS; I ELECTED TO DIVERT TO ZZZZ1. IT WAS DIFFICULT WORKING/COMMUNICATING WITH THE AIR TFC CTLRS ON THIS PARTICULAR FLT/DAY; MY EXPERIENCE TOLD ME THAT AFTER MAKING IT AROUND THE PATTERN IN THE VALLEY COULD PUT US WELL BEHIND THE FUEL CURVE TO SAFELY DIVERT TO ZZZZ1. ADDITIONALLY; CREW FATIGUE WAS ALSO POSSIBLY A FACTOR. ONCE SAFELY ON THE GND IN ZZZZ1; WE LEARNED THAT ACFT HISTORY INCLUDED MANY PROBS WITH THE CTR AND RIGHT AUTOPLTS OVER A PERIOD OF TIME COVERING 54 LEGS OF FLYING. IN THE PAST; ALL AUTOPLT FAULTS ON THIS ACFT; AS PER THE MAINT LOG WERE SUCCESSFULLY RESET ON THE GND.