I'd take a second look at the ignition switch, which is not the key/ tumbler, but is located in the steering column. BTW- I didn't know the Sierra name was available in 1996.

I had a bad ignition switch where I would be driving 70 mph down the interstate, loose all power for a second, and be able to restart the truck without the truck ever stopping forward momentum. Later it would happen at stop signs, all random.