i wanted to know if anyone can help me solve this problem. every once in a while my steering wheel will shake a lot when i press lightly on the brakes. i've replaced my pads, rotors, rotated and balanced my tires & bought new tires, and the problem still exists. it normally occurs at city speeds around 50 or excessively sometimes at highway speeds around 80. but it does not do it all of the time so i think something my be loose. does anyone know what can cause this?

thanks in advance
ps i searched the forums and only found one thing somewhat helpful mentioning pad depostits on the rotor, but if that was the case then it should always vibrate.