Car starts up fine, after i drive it and hit 40 miles an hour it starts shuddering badly, then finally shifts into gear. this all started happening at once i was driving and thought i blew a tire on the highway when i was slowing down. the whole car started vibrating so i pulled over when i noticed a massive oil leak coming from the oil pump drive o -ring, i had that replaced and the car still idles rough and sometimes the rpms just drop to 0 and the car almost dies, with the ac on it sounds like im driving a tractor and runs bad, it also uses alot more gas than it used to before i heard that vibrating noise on the highway. i thouhgt maybe i blew a head gasket cause of the oil leak but the oil level never got that low. i have no idea what the noise is but its loud and it vibrates the whole car.