Bizarre Power Steering Problem - Please Advise

[02_X5_4.6is]

I have a 2002 X5 4.6is with 28K miles and my steering wheel shudders as I turn and drive over uneven road surfaces or go over small bumps like when entering a driveway. It almost feels like I have too sensitive and too much power steering sometimes. I also noticed some feedback on the highway at different speeds, which made it feel like a wheel balance issue or something else going on. My friend who is the head mechanic at BMW drove the car and diagnosed the problem as probably being the power steering belt tensioner or the speed-sensitive-steering control valve. He said it feels like the belt is slipping. However, I do not feel like I’m losing any power steering. So, after changing both the tensioner and the belt I drove the car and noticed an improvement but still a little “tickle” in the steering when turning and going over a bump even at very lows speeds and it seems like it’s more noticeable when turning left. I went back and they then flushed and changed the power steering fluid and replaced it with the synthetic fluid as I was told that BMW issued a bulletin for this solution for many power steering issues. I left the dealership spending close to $600 for the above mentioned work and although the steering feels tighter and the shuddering is a lot less, it’s still there and noticeable, it almost gives like a little “knock” in the wheel when hitting the bump, and a shudder, when it’s doing it.

If anyone has ever heard of this or experienced this I’d really appreciate the help if you know the solution to fix it. I know the 4.6is has a different feel and set-up in the steering than other X5s and it feels a light lighter, so probably not many people have experienced this especially since the 4.6is is not as common as the other X5s. The mechanic suggested I change the speed-sensitive-steering valve next, and if that doesn’t fix it then the worse case scenario is the steering rack. Any suggestions? Thanks in advance.