Once the engine has warmed up, occasionally the car revs up to max for no reason. Blipping the throttle has no effect. The only way to stop this, is to turn the engine off. Once started again, it's fine until it decides to do it again. There does not seem to be any catalyst that causes this as it will happen at any time. It's particularly worrying as the car is being used by my wife to learn to drive and it's also an automatic. I have taken it to a garage and they plugged it into their computer system. No problems found plus they could not reproduce the problem. Ten minutes after leaving the garage the problem started again, so off I went back to the garage but again the engine performed normally. Any ideas?