ACR ARR TO RWY 11 AT OAK DESCRIBED GS ANOMALY CITING DEP ACFT POSITIONING AND POSSIBLE GS FAILURES AS CONTRIBUTORY.

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WE WERE BEING VECTORED FOR AN ILS RWY 11 APCH AS OAK AT 2000 FT. WE WERE CLRED FOR THE APCH WITH A GOOD VECTOR AND NORMAL LOC CAPTURE AND TRACK. WE WERE INSTRUCTED TO CROSS PLAZA AT 1800 FT; WHICH IS ON GLIDEPATH. WE INTERCEPTED THE GS FROM BELOW WITH NORMAL CAPTURE AND DSCNT. AT PLAZA THE AUTOPLT PITCHED THE ACFT OVER ABRUPTLY FOLLOWING A RAPID DOWN GS AND FLT DIRECTOR INDICATION. THE DSCNT WAS STOPPED BY MANUALLY OVERRIDING THE AUTOPLT AND ACFT WAS RETURNED TO PROPER GLIDEPATH. TWR ISSUED A LOW ALT ALERT AND WE REPLIED THAT WE WERE CORRECTING. DSCNT WAS STOPPED AT 1550 FT APPROX 250 FT BELOW GLIDEPATH. COMMAND MODE WAS REENGAGED AND APCH CONTINUED NORMALLY AND APCH LIGHTS WERE SEEN AT APPROX 600 FT. GS AND FLT DIRECTOR AGAIN INDICATED A RAPID DSCNT AT WHICH TIME THE AUTOPLT WAS DISCONNECTED. THE RWY WAS IN SIGHT AT THIS TIME WITH VASI INDICATING 1 OF 2 RED LIGHTS LOW. TWR ISSUED ANOTHER LOW ALT ALERT. WE NOTICED AND RPTED THAT THERE WAS A TRANSPORT SIZE ACFT HOLDING SHORT OF THE RWY IN FRONT OF THE GS ANTENNA. WE RETURNED TO GLIDEPATH WITH NORMAL LNDG ACCOMPLISHED. THE TWR RPTED BACK THAT THEY HAD BEEN HAVING BAD GS INDICATIONS EARLIER IN THE DAY. ACFT SHOULD NOT BE ALLOWED TO TAXI PAST THE ILS HOLD SHORT LINE WHILE ILS APCHS ARE BEING CONDUCTED TO RWY 11. CREWS SHOULD BE WARNED OF THE POSSIBILITY OF BAD GS INDICATIONS IF ACFT ARE HOLDING SHORT OF THE RWY AND AUTOPLT APCHS NOT FLOWN.