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.
From memory I think the Sys80's do have a sleeve. I actually think they are very very close to a WMS sleeve so a new one should not be a problem. I generally kit a full spring kit from RTBB and it makes it like new.
