Recently my car started chugging and "vibrating" at low speeds, and the check engine light is flashing on and off. On inspection, I found oil in the coolant. I don't know a lot about cars, but a somewhat knowledgeable person I know, and this question indicate that it's a sign of the head gasket starting to go. I'd like to be sure, though.
My question is, is the problem almost certainly the head gasket, or could something else be wrong?
EDIT: The engine is a V6.