There is a rod that is between the booster and the master cylinder, if all else is okay with your system. Stops everytime, no leakage, etc, try adjusting this rod. Make it longer, a little at a time, too much and she will lock up on you going down the road , as it gets hotter, the fluild expands, applying the brakes.
You can actually hear this rod hitting the mc if it is real quite. There should be a fraction of an inch clearence. I'd say an eigth of an inch or less clearance.
Jimmy in nc