When I remove the key, it automatically locks up again before the door can be opened?

2000 GM Pontiac Montana 3.4 mileage: 75,000. I am having a problem with my Montana's door locks. When I press the on-lock button to exit the vehicle it automatically locks. Also, when I try to enter the vehicle with the key once it unlocks and I remove the key it automatically locks up again before the door can be opened. When we are driving the locks are continuously locking and unlocking on their own without our contact. We have contacted the dealer for the fourth time and they can't seem to locate the problem. Please note that this vehicle doesn't have a remote door opener. Please advise.