Gear shiftting on its own

I have a 2003 Ford Focus, automatic, with 62000 miles. Just did 60000 miles tune up a few months ago. Recently, when the car is on highway, it will shake and the gear will shift on its own to almost like neutral and wouldn’t accelerate. Had to pull over a few times on highway which was incredibly dangerous. It drives fine in the city.



Any help is highly appreciated.