its called hot spots.. is caused by holding your foot on the brakes when you are at traffic lights .. disc rotors are hot..you are clamping brake pads on them for a continuous amount of time.. the rest of the rotor is cooling down ..but not the part with the pads clapping = hot spots/warped bits.. you need to get those rotors machined strait again..or if they are to thin you will have to buy new ones ..