1999 Ford Taurus Brakes

Sometimes, not very often, when I apply the brakes it feels like I lose 75 percent of my stopping power. I have had new brakes put on and new rotors. They flushed the brake lines and refilled it with fluid and bled the lines. They told me nothing is wrong with it but I still have the same problem. When I step on the brake I can feel a pulsating in the pedal when it happens.