Car Accelerates On Its Own problems of the 2005 Nissan Altima

I was slowly pulling into a parking spot in front of my apartment building. As I reached the front of the spot (the end where the front of my car would be), my 2005 Nissan Altima bucked backward slightly, then suddenly accelerated. The car hit a tree, then continued forward until it stopped in the bedroom of my downstairs neighbor. My foot was on the brake the whole time. I am sure of this, as this is the second time in a year that this has happened to me. The first time, I was pulling into a parking spot in a parking garage, and the car accelerated into a concrete pole. Repairs to the auto body (only) were completed, but it seemed as though the engine was downshifting very hard. I had Nissan check it out, and was told there was nothing wrong. I am not nuts. I am not senile. I am not elderly (52). I have been driving for 34 yrs. And have had an excellent driving record (until now). The car was bought in December 2004, and in less than 18 months, I have had 2 almost identical accidents (same malfunction; same approach to the parking spot).