1993 Lumina ABS Problem

A few weeks ago I replaced all 4 rotors. The pads were still in very good condition, so I did not replace them. After a week and a half I started to feel in the pedal that the front brakes were disengaging and then engaging continuously. I could also hear this going on. The ABS idiot light comes on intermittently. The problem doesn't seem to be too serious; the brakes do not completely disengage. I don't know if it is a caliper problem, because if it was just one caliper the car would pull to one side, wouldn't it? If anyone could help I would appreciate it.

Steve