If its a newer car it could be your throttle body position sensor. (give us more info!)
IAC(intake air control)valve,tps(throttle position sensor)a sticking throttle cable,or vacuum leak.
any car can be easily slowed down by switching the ignition off
not necessarily on a newer car, but if something on the intake system and air is being let in somewhere other than the throttle body, that would cause that as well.
If a car started to speed up suddenly and couldn't easily be slowed down, what part of the car would be malfunctioning?
