I have an unusual problem with my 1981 Toyota Hiace. In the six years that I've owned the car it has started burning sump oil and revving out of control on three occasions. Each time was on main roads, cruising in fourth gear at a sensible speed, otherwise it's pretty random. It starts with a knocking which starts slow and faint, quickly gaining intensity and chucking out a vast amount of black smoke from the exhaust pipe. As it happens when moving it is easy to bring it back under control and to a safe stop. Once stopped it starts up again no problem and we're on our way. I feel that this should point to a specific problem which, so far, escapes me. Any clues would be gratefully received.