steering problem

ive done a search but came to no conclusion as what could be wrong with mine,for some reason when im driving if i go over a bump the steering seems as though it jumps and theres also a fair bit of vibration,i don,t know why it,s doing it.the suspension air springs have been replaced along with the shock absorbers,steering damper and all the suspension bushes,the wheels have been rebalanced and the tracking checked,and it still does it,im hoping that someone on here can point me in the right direction,as the cost is slowly mounting up,and it doesn,t feel stable at motoway speeds,even my local garage is confused any help or ideas would be appreciated.

I have had a similar problem on my Diesel P38, the only problem is I have tried 2 things at once and fixed it so I cant tell you which one worked for me.
Mine felt like the whole steering cradle moved an inch further, laterally, than the rest of the car when going over bumps. Mine has newer wheels fitted than come as standard so If yours does too this would be a good place to start.
I removed each wheel individually and as I did so I rubbed down the mating surfaces with a little glass paper. As I had the wheels off anyhoo I stripped down the brakes and cleaned em all up.
No vibrations at speed now, no death wobble at all, no jumpy bumpy and wicked stopping power.
Try it out Bud its cheap and might just work for ya.

i,ll give that a go tommorow and see what happens mate.