brakes

i have a 95 sunfire with ABS and when i push the pedal down sometimes i hear a squeal the pedal drops and then re-enguages it does it it a sputter mmotion then eventually goes away.i only notice it when im driving at slower speeds i havent tried to lock them up at 60mph not sure if that needs to be done andprolly not very safe.it has done this for about a month now. it doesnt do it all the time it wont do it when the car is stopped, only while driving. every once in awhile the abs light came on. it used to stay on until i replaced a plug by the wheel. so im puzzled thanx for the help for who ever answers.