2015 Subaru Legacy fuel system problems

I am reporting a Sudden Unintended Deceleration event in my Subaru. The vehicle grinded to a halt without driver input. The car stopped in the left lane of a major highway. We put on our hazards and called the state police for help. They were unable to get to us in time. We were hit from the rear. The car's failure created a stationary hazard that led to a high-speed rear-end collision by a Jeep and truck which caused the car to do a 360-degree spin.

Since november of 2017 while driving the car it has intermittently jerked and shook excessively, bucking like a mad horse. Car was brought in to dealer for diagnosis. Dealer was not able to identify the cause. After several visits to dealer where I was assured the problem was fixed the same problem continued. It should be noted that the car, while driving started to shut down and I needed to be towed to the dealer. Dealer claimed the problem was a faulty spark plug. Car was then returned to me.