I would try by passing VATS.
Here'* a site to how to by-pass VATS. http://newrockies.ca/
I had similar problem on my Caddy. Sometimes it wouldn't start and got to the point the Security light would come on. Then I would get a message across the screen saying I had to wait 15 minutes and it would count down. I couldn't get the right resistance from the pellet in the key. So I took the key to a GM dealership to tell me which number I had. Then I used the right resistors, but got the same message. Only part left was the VATS module so I bought a new one. Then the new VATS module had to "learn" the correct resistance. You can't go to a junkyard and get a VATS module because it probably bad or doesn't have the right resistance.
Are you getting a Security light or a message on your instrument cluster?