Cupped tires can also do this and can sometimes be difficult to pin down. Try rotating the tires and see if it makes a change. I had a Mustang in the shop about two weeks ago that needed tires to cure what for all the world seemed like warped rotors. The big clue was that it was not as consistant as warped rotors. We swapped tires from another car to prove what was wrong before the customer would buy new tires.
