can be one of a few things
it could be some slop in the steering box,
the wheel alignment might not be adjusted properly (to much toe in)
not enuff caster (wheel allignment problem)
my celica does it on the highway.. turned out it isnt enough caster... alot of old cars are like that.
drive it.. turn the wheel and if it sticks and pulls the side you turned it ,it could be a tight ball joint and or idler arm as well.