On a new rebuild there will be more blow-by gasses because the rings haven't fully seated. This is more pronounced when driving the car. If you used an aluminum intake you will need to also use the thin paper intake gaskets along with the valley tray. Factory cast iron intakes don't need this. Make sure you're running a PCV valve and/or breathers on both valve covers. You may need to remove the intake and re-seal with RTV.