Is it still stock tire size?
If larger than stock, you may want to increase toe in a little.
The other thing could be slightly bent components. (tie rod/drag link) I'm not entirely sure if you guys have as many options for steering upgrades as us left hand drive folks, but it was night and day when I went from the OE stuff to the Currie steering.
(I didn't think it was that bad, but my wife wouldn't drive it on the highway.)
Another possibility could be the front track bar, but that usually manifests itself with some crazy death wobble.
Good luck.