AIR CARRIER FLIGHT CREW REPORTS GLIDE SLOPE INSTABILITY DURING ILS PRM RUNWAY 27L AT ATL. TOWER REPORTED BACK TAXIING B757 MAY HAVE BEEN THE CAUSE.

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WITH AUTOPLT ENGAGED FOR THE APCH; WE WERE CLRED FOR THE ILS PRM RWY 27L VIA VECTOR (240 DEGS) TO INTERCEPT AND MAINTAIN 7000 FT UNTIL YABBA. LOC CAPTURE WAS NORMAL. GS CAPTURED ON SCHEDULE AT YABBA AT 7000 FT AND APPEARED NORMAL. WE WERE ASSIGNED 210 KTS. THE SPD STARTED TO INCREASE AS I EXPECTED; WITH OUR NEW FLAP LIMITATION NOT ALLOWING FLAP EXTENSION AT THAT SPD. I ADDED SPOILERS TO MANAGE SPD BUT IT SMOOTHLY AND QUICKLY INCREASED WITH A PITCH DOWN AND EXCESSIVE DSCNT RATE (GREATER THAN 3000 FPM). THE DEV HAPPENED APCHING THE DEZEE APCH FIX. I DISCONNECTED THE AUTOPLT AND LEVELED AT 5000 FT IN VISUAL CONDITIONS; ADVISED ATL TWR OF THE ERRATIC GS. THEY ASKED THAT WE MAINTAIN 5000 FT; THEN SUBSEQUENTLY CLRED US TO 4000 FT UNTIL ANVIL AND REISSUED THE APCH CLRNC. THE GS APPEARED STABLE AND ACCURATE BY THIS TIME AND WE FLEW THE APCH AND LANDED; WE WERE IN VISUAL CONDITIONS BY AROUND A 5 NM FINAL. ATL TWR ADVISED THAT AN ACFT; WHICH I THINK THEY SAID WAS A B757; HAD 'BACK-TAXIED ON RWY 27L' AND HAD POSSIBLY INTERFERED WITH THE GS. ANOTHER ACFT ON APCH ALSO ADVISED TWR OF GS DEVS AT THE SAME TIME. LATER WE SAW WHERE WHAT APPEARED TO BE DEPARTING ACFT BEING TAXIED ACROSS RWY 27L ON THE APCH SIDE OF THE RWY 27L GS ANTENNA TO GET TO RWY 28. WE CAN'T BE SURE IT WAS A TAXIING ACFT THAT CAUSED THE PROB; BUT I WOULD LIKE TO SEE TWR HAVE THEM CROSS DOWNFIELD OF THE GS ANTENNA ANYWAY TO BE ON THE SAFE SIDE; EVEN WHEN CONDITIONS ARE VISUAL FOR MOST OF THE FINAL APCH AREA.