Car stalls while going down the highway then restarts immediately and runs fine

I'm going to guess it is the ignition switch or shifter interlock. As you know, the car will not start unless the shifter is in P or N Then when you turn the key, that moves a rod to a switch at the base of the steering column that actually has the electrical contacts to start and run the car. I suspect that the ignition switch is out of adjustment...when you shift to N and then restart it moves the contacts enough to run again.