I had this problem on a nice Mazda I once had. Strangely enough I had just put brand new tires on it as well. The tires were balanced perfectly but it turns out that they were not round. when I jacked the car up and spun the wheel by hand, there was almost a 1/4 inch variation. Apparently this was right from the factory. You might want to check that. I was given a set of replacements.
