Car Accelerates On Its Own problems of the 1998 Audi A6

I own a 1998 Audi A6 which on several occasions has experienced sudden unintended acceleration. While attempting to slow the vehicle by braking for a stop, the engine starts to accelerate and rev all the way to 6000 rpms. The only way to avoid an accident was the depress the brake pedal as hard as possible, put the gear selector in neutral, and turn the engine off. After restarting the engine, the vehicle would operate in a normal fashion. This problem extremely dangerous situation has occurred 3 times since I purchased the vehicle (Audi certified pre-owned) and has been reported to the local Audi dealer on several occasions. They can find nothing wrong with the car.