A319 FLT CREW HAS ERRONEOUS GS INDICATION AND THE ACFT 'CHASES' THE GS ON APCH TO PIT.

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WE WERE FLYING AN AIRBUS 319. WE WERE AT 6000 FT MSL ON THE LOC FOR ILS RWY 28R AT PIT. WE WERE CLRED FOR THE APCH AND 'APCH' WAS ARMED. AROUND +/-23 DME THE GS CAME DOWN RAPIDLY FROM THE TOP OF THE SCALE. GS CAPTURED AND IMMEDIATELY THE GS WENT BACK UP TO THE TOP OF SCALE. THE ACFT FOLLOWED THE GS UP AND AUTOTHRUST ADDED FULL PWR RAPIDLY. AT APPROX +/-6500 FT MSL I DISCONNECTED THE AUTOPLT AND AUTOTHRUST TO GET BACK DOWN TO ALT (6000 FT). NOW ABOVE GS AND HAND FLOWN IN A GRADUAL DSCNT I TOLD FO TO REENGAGE AUTOPLT AND AUTOTHRUST. THE ACFT STARTED CLBING AND ACCELERATING AGAIN. DISCONNECTED; AUTOPLT; AUTOTHRUST; FLT DIRECTOR AND APCH MODE. HAND FLEW BACK DOWN TOWARDS GS. RESET ALT WINDOW TO 4000 FT AND OPEN DSCNT. CAUGHT GS FINALLY FROM ABOUT SOMEWHERE AROUND COFEE FIX; RECONNECTED AUTOPLT AND AUTOTHRUST AND AUTOPLT FLEW REST OF NORMAL ILS TO RWY 28R. I BELIEVE WE HAD A FALSE CAPTURE OF THE GS OUTSIDE THE NORMAL RANGE OF GS INTERCEPT. THERE HAVE BEEN SOME RPTS OF THIS HAPPENING BEFORE; BUT I THOUGHT AIRBUS HAD FIXED IT.