2002 Saturn Station Wagon brake pedal occasionally goes all

2002 Saturn LW200. Twice while driving the brake pedal suddenly went all the way to the floor when braking, although the car still stopped. Each time, it continued this behavior until the car was turned off and sat for a while, and then it returned to normal braking behavior. Dealer could not reproduce the problem. Had the master cylinder and brake fluid replaced, but the problem just happened a second time.