Could be your tires. I notice that my steering wanders every time I rotate my Pirelli's. The tires seem to "set" and track on any grooves on the road. Once they get a bit of wear on them the wandering stops. I'm not sure if this is a symptom of some type of condition or a byproduct of the type of tire. I get the wheels aligned every time I rotate (helps to buy that $100 lifetime alignment at Firestone!). They are usually a little off, but not enough to explain what I'm experiencing.

-- Best, John