Ya i had the head gasket changed a tune up put on it. new plugs new air filter. new oil new gas. Could it be the cheveron supreme doing that. I mean its suppose to use supreme gas, so I wasnt really factoring that in. I just dont want to go to a test and repair and them try to tell me its the most expensive thing before I get some opinions. I know the cat is a very real possiblity but I hear they are around 1,000 dollars and you cant put an after mrket so Im hoping to hear its not the cat.