4.6 GT, oil in last cylinder spark plug hole

[jfarinha]

the car was bucking, the engine wasnt running rough. just bucking, especially under hard acceleration. I changed the plugs, it ran great. the last plug on each side, toward the engine compartment, were covered in oil. I put in new plugs, it ran great. for about 15 minutes, then, started running crappy, and bucking again. I took out the plugs, and it looked like, I dipped them in an oil pan, they were just over, and it was mostly on the tops of the plugs. I would think, there was a head problem, or could it be an intake problem. I have never seen oil in a spark plug hole. the car has over 100k on it, its a 1996 Mustang GT.
also, I was thinking about replacing or rebuilding the engine, can I, install a later model engine, like an 05. I want an engine with low miles, and tranny. and what would you need? computer?