Ok a couple things you should check for when this is happening. 1. Is the security light on when you try to crank it and it wont start. That alone will tell you if it is security!! 2. Try using the other key almost immediately. Don't give it time to reset or whatever it's doing that cases it to start out of nowhere. 3. Have you ever heard of something called identity theft?? It is a little security system that GM was doing for a few folks for free for a while with new purchase and some service vehicles. Anyway, if you have Identity Theft, there will be a little keychain thing on your key ring. IF the battery dies in it, you have to physically press the button on the key chain(The button does not look like a button till you press it and the light lights up on the key chain deally.) 4. Check the wiring of the Keyless start, they could've tapped into a line or cut something else that is now giving this truck an intermitant signal through the wire!!! Especially grounds! 5. Last thing I can think of right now is if you checked or changed you sprkplugs lately??