I have a Nissan Maxima that has been lugging on hills as well as level grade roads. Spark plugs were changed by a friend approximately 7 months ago. There has been a continous oil leak around the head gasket since that time. I found 1 plug was fouled out so I changed the gaskets on head and replaced all plugs again. Any suggestions as to what else it could be as it is still lugging? Would living in Hawaii have any thing to do with it like living in Colorado?