Do you feel any feedback in the brake pedal when it jerks? Unless you have a surge of brake pressure coming from somewhere, the brakes don't just jerk. Check out the FSM in the sticky. It will tell you how to diagnose brake issues. First things I would check out are your brake master cylinder and your brake booster. If neither of them show any signs of trouble then I would look elsewhere. Do you have an automatic transmission? If so you could have an issue with your tranny.
