Car Accelerates On Its Own problems of the 2002 Ford Mustang

First incident: I was approaching my mail box to stop and get my mail out, while braking instead of stopping the car accelerated and went right past my mailbox. I drove into my stone driveway, the car continued to accelerate rapidly even though I had stopped, the tires were spinning so fast they mad ruts in my driveway. Even sitting there in park, the rpm needle was racing upward. No check engine lights go on. I turned the car off. Next day I went out, and car was still accelerating without me stepping on the gas. Scared to drive it, I had it towed to a service center. They had it for a week and a half, never happened for them, and said unless it happens while they have it, could not determine the cause. (although they did mention a few possibilities: throttle position sensor, leakage, computer) as I was taking my car to a Ford dealership on 8/9/13 for other service, I explained what happened and they said also unless the car did it while it was there they would not be able to check it out. Second incident: while driving on a highway, probably about 55-60 mph, the car on its own sped up, the rpm need seemed stuck at 3, even though I was breaking the car wasn't slowing down; and just like that, probably in 30 seconds, it recovered and ran fine. I wrote a letter on 9/6/13 to Ford motor CO explaining what is happening and asking for their help in identifying a solution. I received a letter back from Ford motor CO dated 9/16/13, they offer no assistance at all.