like they said, it could be warped rotors. If you apply a consistant amount of pressure on ur brake pedal and your brakes seem to grip, ungrip, grip, ungrip..almost like your brakes are only grabbing certain points or even like a slow motion ABS, then its more than likely warped rotors but if applying consistant brake pressure results in a smooth stop then its something other than warped rotors.
hmm..hope that bunch of gibberish makes sense