If your check engine light doesn't turn on when your driving, then there are no codes to scan so you can save your $$$s on that. I would check the intake for obstructions/leaks and clean up the throttle body and EGR passages if this hasn't been done yet. The plugs and wires are probably pretty well worn by now also and could be replaced. A bottle of injector cleaner over your next couple of gas fill-ups wouldnt hurt. An upper/lower manifold overhaul brings new life to tired contours.
