I had had exactly this problem.

I changed the lower suspension arms, droplinks and track rod ends. No cure.

In the end I changed the steering rack which did cure it.

It's quite a common problem and if you look on utube there is a video of it;

http://www.youtube.com/watch?v=ATjdVu1Ff-Q

Not my video but my car did exactly this but in the other direction.

Swapping the rack out means dropping the subframe and isn't that difficult a job.