1997 Chevy Pickup
 

I've got a 97 Chevy Silverado pickup starts great drive down the road, and the motor just shuts off, floor gas and truck restarts automatically, without touching the ignition switch! WTF??
RPMs go to zero
Automatic transmission
Vortec engine
All electronics continue to work( lights, radio, and gauges)
This happened 6 times on the interstate driving 30 miles @ 70-75 mph.
Did a full tune up last year.
Never seen or heard anything like this so any help would be greatly appreciated.