Steering wobble/violent shake!

[passatawx]

Hi everyone! I was driving down the M5 today, cruise control 72mph for about 1 hour. I had to brake/slow down because of the road works when suddenly the steering started to shake violently. Pulled over on the hard shoulder and could see nothing wrong. Wheel bolts were tight, fluid was ok! Stopped at Michaelwood services for about 40 min and set off again! After another 50 miles same thing again. Is my steering rack gone? Or the pump? Help please! Cheers.