I hate intermittent faults!  

Around 40-45 mph, I have experienced a worrying small oscillation in the steering wheel at a frequency of around 5Hz.  It has happened maybe around ten times during my ownership.

It seems to disappear when the speed increases slightly.  Despite my best efforts, I have not been able to induce it.  It lasts for around 5-10 seconds.  Hard acceleration, hard braking, violent weaving - no result.

The first occasion, I assumed the wheels were about to fall off.  They were not.

I thought maybe the assistance was electric but, a glance under the bonnet, proves it is hydraulic (and, yes, the reservoir level is correct).  It is not clear whether the steering actually judders or whether it is just the wheel.

I am no whizz on modern cars (although have '79 MGB V8, and a couple of BMC 1.5 marine diesels under rebuild) and have no clue what the cause could be.

Anyone with a similar experience please?

It will be worth checking the torque of the ball joints on the front suspension, they might be loose. You don't say if yours is an early Jag, those ball joints 'point' downward. But I don't think driving is advised, in case the car falls to bits.

hi

have you tried swapping front wheels with back,

have known a bad tyre to cause this a certain speed

cheers

Joe

I agree with Joe one of your front tyre's could be a little oval one check you could do is

jack the front up so the just off the ground and spin the wheel if it touches in one high spot 

that could be the problem ????

I took the car into my local garage for a suspension and tyre check but they found nothing untoward.

It would be easier to diagnose if the problem could be invoked but, try as I might, I cannot.

Could be the old problem of balancing the front wheels. In the old days it happened quite often as the tyres started to wear and required rebalancing. Dave