i was driving my car one day..i heard a thud under my car. i then lost power to my car. it sometimes shifts into Second on it's own..but most of the time i let off of the accelerator to shift from one gear to another. when i try to 'floor' it, it bogs down, then goes slow.i let up..then it moves ..the best speed i get is 45 mph on a straight a way.. or i pick up speed going down a long hill.sometimes the weather affects it alot too.. too hot, and it moves slow..slightly cooler and it moves, but not at the speed limit . i have had several things done..nothing yet seamed to work..any ideas? PLEASE HELP!!!