Car Accelerates On Its Own problems of the 1999 Mercury Mountaineer

While stopped at a red light with my foot on the brake pedal, the engine bucked and misfired as though it was going to die. Suddenly, the engine revved to the point that with both of my feet on the brake pedal, the car crept ahead and tapped the car ahead of me. Just as quickly as the engine surged, it dropped back in revs to normal idle. On my way home from the scene, it occured again, except this time I had my hand on the auto gear change, moved it qiuckly to neutral, and turned off the ignition. I made it home without further incident, called the dealer on Monday , and had them tow the car to the dealership as I did not feel that the vehicle was safe to drive. After having the car for two days now, they claim that they can find nothing wrong. I told them that I would not accept the car back without a specific reason for this sudden acceleration. They say there is nothning wrong that they can find, but I will not drive this vehicle until a fix is located and repaired.