Perhaps the tires had been rotated just prior to you buying the vehicle....is that a possibillity? If so, perhaps what happened is "radial pull" - when a seemingly perfectly good tire only wants to ride well in one location on the car.
I had a van - bought it w/ 45k miles on it. I rotated the tires at 55k. Got a major pulling at higher speeds only. Rotated them back, it went away. Perhaps your'* is a lessor degree of this radial pull...but if you talk to tire guys, they say there'* no such thing. But I believe!
At any rate...hope the rotating did the trick!