A B737-300 EXPERIENCED A GLIDE SLOPE ANOMALY WHILE CONDUCTING A COUPLED ILS TO RWY 11 AT OAK IN VISUAL CONDITIONS.

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ALTHOUGH IN VMC CONDITION; I ELECTED TO FLY THE ILS COUPLED WITH THE AUTOPLT WHILE MONITORING IT ON THE HGS. THE ILS SIGNAL WAS VERY ERRATIC ON LONG FINAL. IT SEEMED TO SETTLE OUT CLOSER IN. PASSING ABOUT 2000 FT AGL; THE ACFT BEGAN A DSCNT TO APPROX 7 DEGS NOSE LOW. THE GS SHOWED CENTERED. PASSING ABOUT A 2 DEG GLIDE PATH; I DISCONNECTED THE AUTOPLT AND MANUALLY FLEW THE ACFT TO A PROPER 3 DEG APCH PATH. THE GS VARIED UP AND DOWN THROUGHOUT THE APCH. I QUERIED OAK GND CTL AFTER LNDG; AND HE ACKNOWLEDGED THAT THE GS WAS ERRATIC DUE TO OBSTRUCTIONS AND BUILDINGS (EFFECTIVELY; NOTHING HE COULD DO ABOUT IT). ON TAXI OUT; HEARD ANOTHER ACFT COMPLAIN WITH THE SAME COMPLAINTS WE HAD. THE ILS WOULD HAVE BEEN UNSAFE IN IMC CONDITIONS. I HAVE PREVIOUSLY FLOWN COUPLED ILS RWY 11 APCHES WITH NO GLIDE PATH PROB. THIS SIT NEEDS TO BE INVESTIGATED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT IN 25 YRS OF FLYING HE HAS NEVER SEEN SUCH A LARGE CHANGE IN A GLIDE PATH. WITH THE GS CHANGE; THE 2000 FPM DSCNT TOOK THE ACFT INTO THE VASI RED OVER RED IN A MATTER OF SECONDS. OAK GND CTL ADMITTED TO THEM THAT OTHER ACFT HAD NOTED THE CHANGE BUT THE TWR COULD NOT CTL IT. ANALYST QUESTIONED WHETHER THE ANOMALY WAS IN THE VICINITY OF THE NEW CRANES. THE RPTR SAID HE WAS NOT FAMILIAR WITH THE CRANES BUT THE ANOMALY OCCURRED INSIDE OF THE BAY BRIDGE. THE RPTR DISCUSSED THIS EVENT WITH THE ACR SAFETY PERSONNEL.