2014 Chevrolet Other Captiva Sport LTZ 6000 miles only!

We own a 2014 Captiva Sport that has a very unusual problem. I only happens once in a while. Out of the blue the car will just quit when traveling down the highway. And I mean everything goes off, all the dash lights, all electrical, If you put the car in neutral while coasting, turn the key off and it will start up and you can drive on. Most of the time it happens in the driveway like today. I went to get into the car, I hit the key button to unlock the door, nothing happens, so I unlock the door manually and get in the car, turn the key in the ignition and nothing, nothing lights up on the dash at all everything is dead except the electric seat. So I turn the key to the on position and adjust the seat. The lights on the dash all come on and I can start the car and drive it for days and then out of the blue it will happen again. I have had it back to Chevy dealer twice now and they can't figure it out. Please help, we really like the auto!
Thanks
Carrot