Its all to do with suspension geometry when cornering. This explains it fairly simply and saves me typing paragraphs on it! https://suspensionsecrets.co.uk/ackermann/

Pretty much every modern Porsche (maybe not the SUVs) has this and the forums are awash with new owners wondering whats wrong with their cars!!

 

Hi there you might find out that you need your disks machined it will cause the steering to shutter

 

I do not believe this is the issue otherwise it would be noticeable and consistent at near max turning radius.
It would be easily repeatable.
I think in the case of the X150 this is primarily a programming issue with the load from the power steering pump, engine RPM, and power steering demand getting out of whack.

If it were a lack of ackerman geometry, you'd notice it even with the car in neutral while you revved the engine.
In my Z06, the tires are still shuddering from the lack of positive ackerman geometry and LSD even in neutral, it isn't power steering. The shutter is the tires skipping.
If I put my XK into neutral, keep the engine RPM up to 1,500-2000RPM, and make a slow tight turn it is quite smooth; aside from extreme cold where the tires are being asked beyond their grip, but this "skip" would be there if I asked beyond their grip even not at low speed and a more normal radius as the skip would be the tire temperature flashing in and out of grip.

So, do an experiment where you take a low speed down hill at max turning radius while in neutral, and keep enough RPM that guarantees the power steering pump is adequately fed.
If the shudder is still there, then it is likely a combination of your alignment and tires.
It may be that what you are noticing is much more minor than what I think the issue of the thread is.
I no longer have it happen as a regular thing since changing the fuel filter in 2016.

To me, it is very much a power steering programming issue where what it is trying to accomplish and what can be accomplished are not matching in some circumstances.

 

I'd have to say thats just not right. Call up a geometry specialist that you trust and he'll tell you all about it.

 

Experimenting with different situations to find situations when the shuddering is most pronounced or absent can only help to determine what is causing it. I'm sure Jaguar has had many complaints about this and knows more about what is causing this than they are saying. Maybe there is a bulletin out on this--there should be.