Seems as if my car went into neutral

It all started when one day I was driving on the highway going about 70mph then all of a sudden it seems as if my car went into neutral. I pushed down on the gas peddle and nothing happened then all of a sudden a big clunk happened and it drove fine for about 15 seconds then it did it again. It happened about 10 times and then the check engine light went on and then the car drove perfectly fine. I took it to a mechanic and he said it should be fine, and didn t explain anything to me. Now every time I drive it it seems to happen after I ve been driving for 10 min or so. About a week Later my car is now just shaking so badly I feel as if its going to have parts fall off, and again I took it to another mechanic, and he said it should be fine nothing to worry about. Now another week later I have a coolant leak. I don t know what I should do I don t want to take it to someone and have them say, oh it should be fine. If you could please give me an insight about what s wrong and the price range to fix the problems. Thank You