[73 Mustang Boy]

my fiance has a 95 tracer with 140 thousand miles on it. It has recently started acting up. When going down the road at a constant speed the engine will stumble for a few seconds and then resume normal operation. The car also seems excessivley sluggish even for a 4 cyl. Other than this the car runs and idles fine and gets good gas milage. I ran a bottle of fuel injector cleaner through it but that seemed to make it worse. The only thing that I can think of that could cause the problem is one of the spark plugs. The car has three new plugs and wires but one plug has not been changed in at least 5 years and still has an orginal plug wire on it. Does anyone know if the plug/wire could cause this problem, or have any other ideas as to what might be wrong? I am willing to do almost anything to fix the car and any help is greatly appreciated.

Thanks, Justin