1983 Buick Skylark anti-lock brakes

1983 Buick Skylark mileage: 18,000. The anti-lock brakes seem to engage right as the car nears a stop. There is a sound as if a valve is applying and releasing pressure and at this time the brakes grab then release at there own will. I have been told by a dealer that there are three parts to this anti-lock system and all have been tested and were fine. He said the rotors were warped, turned them and the car worked fine for about 000 miles and then started again. Placed new rotors on this time but the problem still exists. It does not happen all the time but enough to always make you wonder. Other mechanics have said the whole anti-lock unit needs to be replaced. Brakes work fine at braking at high speeds, it only occurs nearing a stop but its making the car feel unsafe to drive. Any advice would be very helpful and thank-you for your time.