2001 Mercury Sable engine problems

My 2001 mercury sable accelerated as I entered the highway. The car would not stop even when I applied the brake. I called 911. The 911 operator suggested that I apply the parking brake which I did. I went about 15 miles riding the shoulder going between 60-70 miles per hour. I eventually exited the highway but could not stop even at the end of the exit ramp. I made a right hand turn. At this point I had reduced my speed to approximately 30 miles per hour. The car eventually landed in a ditch.

2001 sable with 57000 miles. Vehicle will start, kill and then cannot be restarted for at least 1/2 hour. Sometimes will not start at all. Service people have hooked up to sensors but cannot find what is wrong. Problem is sporadic, happening about 1 to 2 times per week. Scoped fuel pump but said it was working properly. Said fuel filter and all other systems checked out ok. Don't know what to do next. Feel uneasy about driving the car anywhere far from home.