Serious steering shudder!!!

Help!!!
Going along the M4 last night the cars steering started to shudder as if I was on the rumble strip.It got that bad I had to pull onto the hard shoulder!.Checked to make sure that all wheels were not loose and after mooching down the hard shoulder it cleared.Ten minutes later the shudder returned and became worse,the steering wheel was trying to kick to the left as if the wheel was ready to come off!
The AA man(cheers Ed) jacked up all the four corners in turn and after a lot of checking said he could not find anything wrong.The only thing he could find was a possible worn bush which he said was not that bad to effect it.
Carried on slowly without problem only to have to go into Reading services with the wheel seriously kicking over to the left.I thought the dam thing was trying to kill me!!.
How bad it felt and shuddered I thought there is bound to be something loose,but no all was as it should be.
Limped back onto the motorway and from  Reading all the way to Cirencester the car ran without a problem???
Have not got a clue what it could be but I know there's something serious which comes and goes.
Any ideas?

did you notice if it only does it at a certain speed
you could have a slight buckle in your wheel which runs ok until you get to a high speed….try swapping one wheel at a time with the spare and run it upto the speed you know will shudder

an un balanced wheel will cause this but not normally as bad as you say but a buckle in the wheel will

When it started I was sat at 70,but it continued until I came to a stop.The AA man checked all the wheels with no problems.What I can't work out is that it violently shudders and kicks one moment and then the next it runs perfect?.Could it be a intermitant fault with the steering rack?

cv joint locking up? :dontknow:

I've just had a similar problem with my toyota lucida. Basically there was a slight warp on one of the discs and the caliper started ceased up, would create major vibrations and pull to one side. Once I had stopped for a bit and let it cool down it would be ok again for a while untill it got too hot again, and as the caliper was not always siezing up would be ok for some time and then just happen all of a sudden. i would generally notice it happen more on longer runs.  Not saying this is your problem but the symptoms just seem very similar to what I have recently experienced.

Dan

Cheers Dan,I think you may cracked the problem.The car ran fine back from Cirencester(119 miles) without a problem,well the speedo shows 70 but I'm really doing 62!,that's for another time.
Thanks again for you info.

Sean