525tds steering

My bmw 525tds has 190000 miles on the clock, about 8 months ago I noticed it was pulling to one side , i had the tracking checked and found nothing wrong, it got progressively worse and there was a looseness and jerking of the wheel as i went over a speed bump, had it checked out and found nothing wrong. i had a few of the steering joints changed but it is as bad as ever and the feeling of looseness persists. Any ideas.

I have a 525tds with 156000 on the clock and had similar symptoms until I changed the upper track control arms & bushes. The bushes had previously been classed OK by a BMW mechanic, I did'nt believe this and changed them anyway with complete cure of the knocks etc.. The bush rubber has only a very small movement before metal to metal contact occurs and it is therefore difficult to tell when they are due for replacement. Another possibility is the steering box requires adjustment.
Hope this is helpful,
Cheers