WHILE DRIVING ON AN EXPRESSWAY I ATTEMPTED TO SLOW MY VEHICLE. THE BRAKES BEGAN TO ENGAGE BUT THEN I FELT THE CAR ACCELERATE AND THE CAR "STUTTERED". IT MAY HAVE BEEN THE BRAKE ASSIST KICKING IN. IT ACCELERATED. AFTER A FEW SECONDS, THE BRAKES FINALLY ENGAGED. ABOUT 2 MILES DOWN THE ROAD, THE SAME THING HAPPENED EXCEPT THAT THE BRAKES DID NOT ENGAGE UNTIL A FEW SECONDS LATER AND I REAR-ENDED SOMEONE. THANKFULLY THERE WERE NO INJURIES.