03 Audi A4 Cab Starts and stops

For the last year I have been having problems with my car starting up and shutting down. Sometimes it starts up within a few minutes sometimes not. I had it towed into dealer 2 x in last 2 months. Sept 08 they replaced the ignition lock cylinder and they just replaced it again 5 days ago. yesterday i went to start it up and it shut down. Are they not looking far enough into the problem? I am afraid to drive the car. it only has 90k miles and it should run better than this.