AN S80 PIC PERFORMS A GAR AFTER A DESTABILIZED APCH AND HE FINDS HIS ACFT IS NOT ALIGNED PROPERLY FOR LNDG ON RWY 28R AT PIT; PA.

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ATC FACILITY: PIT TWR. APCH TO RWY 28R RESULTED IN GAR FOR FOLLOWING REASONS: INSTRUCTED TO MAINTAIN 210 KTS AND WHILE SLOWING AND CONFIGURING WENT HIGH ON THE GS IN QUARTERING TAILWIND AT HIGHER THAN NORMAL RATE OF DSCNT (1400 FPM) AND IRREGULAR MOVEMENT ON RADIO ALTIMETER CAUSED BY TERRAIN UNDULATIONS. THIS SHOWED HIGH RATE OF TERRAIN CLOSURE AND LOW RATE OF TERRAIN CLOSURE. I WAS IN MAP MODE FLYING OFF THE ADI BECAUSE THE ILS HAD NO ILS/DME TO SHOW ME DISTANCE FROM THE RWY. TWR CALLED AND ADVISED THAT WE WOULD GET GS FLUCTUATIONS DUE TO AN ACFT XING THE RWY. WX WAS RPTED ABOVE 800 FT AND 2 MI BUT IN REALITY WE BROKE OUT OF THE CLOUDS AT 500 FT. THE GS DEV WENT FROM FULL SCALE UP; TO FULL SCALE DOWN; THEN SETTLED ON 1 1/2 DOTS HIGH. THIS COUPLED WITH THE ERRATIC RADIO ALTIMETRY WAS SOMEWHAT DISORIENTING. FIXATING BRIEFLY ON THE GS; I DRIFTED L OF CTRLINE. USING THE ADI FOR ILS GUIDANCE WORKS WELL UNTIL YOU GET A BIT L OF THE LOC; THEN THE FULL ROSE DISPLAY IS THEN A MUCH CLRER REPRESENTATION. AS WE BROKE OUT OF THE CLOUDS; WE WERE NOT IN A GOOD POS FROM WHICH TO LAND. I EXECUTED A GAR AND SUBSEQUENT APCH AND LNDG TO SAME RWY. A LOT HAPPENED IN A SHORT TIME AND THE WHOLE EXERCISE WAS VALUABLE. WOULD HAVE PREFERRED IT IN THE SIMULATOR.