You may want to inspect the intake manifold to make sure that it is not faulty. I was having repeat problems with intake manifold gaskets getting torn and leaking
oil into the cylinder and the last time that it was getting replaced my mechanic noticed that my RPM intake manifold had a slight twist to it and would teeter corner to corner. Bought a new intake manifold and the problem has never resurfaced again. Every now and then you will get a bad casting no matter who the manufacturer. If your intake manifold is not sitting square it will lead to problems.
Otherwise, I would just follow the advice given above.