My SX does exactly the same. (you should never drive over 70 anyway...) When i was driving to kent, it started. I have yet to get my tracking and balancing done. another suggestion has been to make sure your tyre pressures are equal, if that has not already been done. I shot the A24 (to Worthing from Horsham) and got up to 95 (SX top speed 83), trying to go round the bends at that speed with vibs was difficult i tell thee...
Adam