little black box causing grief

Hey peeps. I have a 98 slt with the 5.9 magnum. On the alternator is a little black box. my first thought was voltage regulator. Now I understand it is something else. anyhow the voltage will drop on the vehicle but comes back when tapping on the little black box. said box is part of a wire harnes from the positive batt cable. What I would like to know is, what is this box exactly and does anyone know of similar problems, what fixes them, and where to procure this elusive part. any suggestions are greatly appreciated. thanks fer lookin!!