Good luck finding a knowledgeable Z mechanic in our area. I've heard good things about "Fighters Garage" in P-cola but i've never been there. I dont trust anyone but myself when it comes to my car. When you changed plenums maybe the upper to lower gasket didnt get replaced properly. Maybe the iacv gasket on the back of plenum is leaking after changing it over from old to polished plenum. Have you done a vacuum check? I always reference this site