Starter problems of the 2008 GMC Envoy

I wil be going down the road and my car would just shut off on me. I would have to keep trying to start it and after a few times trying it it would start. I have to keep turning the key over and over a few times to get the ignition to start. It has stalled on me 5 times. Some times it will crank right up and other times I have to keep turning the key over and over for it to crank. When I turn the key it doesn't do anything no sound at all. But if I keep trying it it would start up. I had the starter and battery checked but those were fine.