A B737-800 FLT CREW EXPERIENCES ERRATIC AUTOPLT BEHAVIOR ON THE ILS DME-2 RWY 1 APCH TO MGGT.

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I HAVE MADE THIS APCH 8 TIMES IN THE LAST SIX WEEKS. FOUR WERE IN VFR CONDITIONS; AND THE LAST 4 WERE IFR. EACH TIME THE AUTOPLT AND OR FLT DIRECTOR BECAME ERRATIC AND OR UNSTABLE ON THE APCH AT THE FAF. ALL APCHS WERE FLOWN EXACTLY AS PUBLISHED WITH THE ACFT CONFIGURED WITH GEAR AND FLAPS 30 AT THE 6 MILE FIX; WHERE THE TURN TO FINAL IS STARTED AND A DSCNT FROM 6600 TO 6300 IS STARTED. A 30 DEG BANK TURN PUTS THE ACFT OVER VILAN; WHICH IS ALSO THE GLIDESLOPE INTERCEPT ALT. EACH TIME THE ACFT HAS HAD TROUBLE INTERCEPTING THE GLIDESLOPE OR THE LOCALIZER OR BOTH. SEVERAL SCENARIOS HAVE RESULTED. SOMETIMES THE FLT DIRECTOR FAILS. SOMETIMES THE AUTOPLT DISCONNECTS. SOMETIMES BOTH. BUT EVERY TIME THE AUTOPLT MUST BE DISCONNECTED DUE TO ERRATIC BEHAVIOR AT GLIDESLOPE CAPTURE. ON THIS LAST ONE; THE AUTOPLT PITCHED THE ACFT 15 DEGS NOSE DOWN AND PULLED THE THROTTLES TO IDLE. IT WAS DISCONNECTED AND A RECOVERY TO THE GLIDESCOPE WAS MADE BEFORE IT WENT FULL SCALE; AND A LNDG WAS MADE. HAD THIS BEEN MY FIRST APCH INSTEAD OF MY 8TH; I WOULD NOT HAVE BEEN EXPECTING A BUMP INTERCEPTING THE GLIDESLOPE; AND TRAGIC RESULTS MIGHT HAVE OCCURRED. I PERSONALLY BELIEVE THIS APCH REQUIRES MANEUVERING THAT EXCEEDS THE ABILITY OF THE AUTOPLT AND FLT DIRECTOR. IT NEEDS TO BE TEST FLOWN TO DETERMINE IF IT SHOULD BE A VALID APCH FOR THE 737/800. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THE ARPT IN QUESTION IS GUATEMALA CITY; GUATEMALA (MGGT). THE RPTR ADVISED THAT SEVERAL OTHER FLT CREWS IN HIS COMPANY HAVE RPTED EXPERIENCING THE SAME DIFFICULTY WITH THE ILS DME-2 RWY 1. NO PROBS WERE NOTED WITH ANY OTHER INST APCHS AT THIS ARPT. RPTR STATED THAT HIS ACR SAFETY DEPT HAS DETERMINED THAT THE ANOMALY IS NOT RELATED TO THE ACFT OR THE NAVIGATION HARDWARE OR SOFTWARE.