when you say tacho sensor do you mean the 'tach generator' on the steering wheel? or the steer position sensor on the drive unit?
i had a similar problem with a truck and it turned out that the 'tach generator' on the steering wheel was not generating a good signal of 5v or more turning the wheel normally. This was causing the steering to malfunction because of the low voltage output even while turning the steer wheel fast. I went through the same troubleshooting problems as you by replacing the eps control and position sensor and all sorts of checks and adjustments before we figured it out consulting with a factory engineer.