Intake manifold Gasket leakage is ,, I cant say a common problem,, but it does occur.. If it does happen,, It WILL cause a misfire. Easiest way to diagose is to spray Brake clean, carb cleaner, while your car is running along the area where the intake meets the Cylinder head,, top and bottom. if you notice any RPM change while spraying,, Its leaking. Thus,, a bad thing.