Whatever you do - don't lubricate the plunger. Replacing the spring and giving it a clean should do the trick.
+1 - Just took the plunger apart the other day on a game that was not shooting well - full of grease. Good clean and works beautifully. If it has a sleeve (can't recall on Gottlieb) that is the easiest way to fix - clean the rod and replace the sleeve and springs if necessary.
