Did you check the rack and pinion, my wifes 2006 Ford Fusion has the same issue. I am about 99% that is the problem on hers. Because it is no always consistent. I work in the tire and wheel industry (15+ years now) so I have balanced them many times and know it's not the tires. I have installed new tires and the problem persists. I have replace the front struts, upper and lower control arms, ball joints, tie rod ends, pads, rotors, and steering linkage. So pretty much the entire front end but the steering box/rack and pinion. So that has to be it.