1998 Chevy Lumina braking problem

1998 Chevy Lumina 3.1 mileage: 96,000. When I press the brakes to slow down the car will slow in a lunging fashion. I replaced the front brakes and rotors about 2 years ago because the rotors were warped. It feels like the same thing all over again. I have 4 wheel disk brakes and when I apply them with medium or hard pressure the car stops great. I don't know if this has anything to do with it, but I have dust shields on all four wheels. Other than that, the car is stock. Any ideas on what might be causing this?