You are most likely experiencing a failed or failing Antitheft security system. Gm has had issues with this system since they first installed them in the early 80s. Contrary to what others may tell you, this system CAN be bypassed and eliminated. People will tell you that it is impossible, or even worse, illegal. This is simply not true. It is your vehicle and you can do whatever you want to your vehicle. This site has a lot of information and can help you beat this problem for far less than the $1000 dealer quote.