Why my car stops accelerating while driving?

have a Ford Fussion 2007. I was driving at about 40MPH and my car suddenly stops moving like if it was on neutral it accelerate by it self wth out moving even though it was on drive. I pulled over, turn it off and started again and it took me back home normally but now it did it again I took it to a mechanic and he put it in the computer but he says nothing is showing it seems that everything is ok but that car now is not moving! It only goes on reverse or low driving but not drive

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