A HAWKER 1000 ON AN ILS APCH TO SWF OBSERVED SEVERAL LOC SWINGS AND EXECUTED A MISSED APCH. THE PRECEDING ACFT WAS IDENTED AS A C5.

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WE CROSSED THE FAF ON AN ILS APCH AS A C5A CARGO JET WAS LNDG. LOC WAS OPERATING NORMALLY. AT ABOUT 1000 FT AGL THE LOC NEEDLE STARTED TO MOVE RAPIDLY FROM SIDE-TO-SIDE AS THE AUTOPLT WAS COUPLED AND ATTEMPTED TO LOCK ON THE SIGNAL. THE ACFT STARTED TO OSCILLATE. I DISENGAGED THE AUTOPLT AND ATTEMPTED TO HAND FLY; BUT THIS WAS FUTILE AS THE SIGNAL WAS ERRONEOUS. I EXECUTED A MISSED APCH AND REQUESTED ANOTHER ATTEMPT. TWR SAID THE LOC WAS INDICATING NORMAL. I ASKED IF THE C5A WAS IN THE ILS CRITICAL AREA. WITH VISIBILITY SO LOW; TWR WASN'T SURE; BUT THOUGHT HE MIGHT HAVE BEEN. THE RAMP AREA FOR THE C5A'S IS VERY NEAR THE RWY. I CAN ONLY ASSUME THE C5A WAS CAUSING LOC INTERFERENCE WHILE TAXIING OR DOING AFTER LNDG CHKS UPON CLRING THE RWY; AS OUR SECOND ILS APCH WAS NORMAL WITH PROPER SIGNAL INDICATIONS. MANY PLTS ARE AWARE OF THE ILS CRITICAL AREA UPON TAXIING TO A RWY FOR TKOF; BUT FORGET ABOUT THIS AREA UPON LNDG. WE NEED TO EMPHASIZE THIS FACTOR; AS WE CAME VERY CLOSE TO BEING AN ACCIDENT.