Originally Posted by B&W_Evader

Oil in the manifold sounds really bad. Only 2 ways I can think of for that one. Over oiling the air filter or too much blow by in the block. Blow by would be the worst, happens from scratched piston walls or messed up rings. You could do a compression test while you have the spark plugs out. I'd guess it should be somewhere around 160. All pistons should be within 10psi range. Also look at your spark plugs. They should all look the same. A bad cat should not be intermittent. None of the really major issues should be intermittent. Good luck!