2001 Steering system problem

My 2001 Seville has a "catch" in the steering when turning left at low speed. It is more of a feel than something audible. So far the dealer has replaced the steering rack, the power steering unit, and the lower sector shaft, all with no improvement. I have logged 20 days in the service department. Has anyone else had a similar problem? I have noted several tech bulletins concerning tie rod problems in this model, however it doesn't feel like a tie rod problem.