last night I drove to Bristol. Just as i was leaving the M32 to enter Bristol my steering wheel started shaking hard leading me to believe i had a flat tyre, pulled over on to the hard shoulder to check it out and couldnt see anything wrong, checked the tyres and checked all the bolts but everything was ok. I decided the hard shoulder wasnt the best place to be and considering i couldnt see anything wrong i pulled off to find a more sensible place in bristol to park and investigate. When i pulled off the car was driving fine, the 5 minute break had stopped the shaking for some very odd reason. I decided to continue to Krispy Kremes as the shaking had stopped and they have a big car park where i could check it over properly, half a mile into my journey it started doing it again so found a sensible place to quickly pull over, jacked up all the wheels had a look underneath, checked all the bolts again but nothing out of the ordinary, so once again we decided to shake our way to krispy kremes to ring the RAC there and eat doughnuts while we waited. just approaching Krispy it suddenly went berserk again trying to drag me onto the right hand side of the road. Pulled into Krispy Kremes and rang the RAC. It took the RAC man about an hour to arrive, he drove the car round for about 10 mins to try get it to happen but it was driving as smooth as it ever had, he checked underneath couldnt see anything so told us to drive home slowly and ring them if it happens again. low and behold on the m4 on the way back luckily just as we were exiting for the A46 it started doing it again alot worse then it had before, i had to fight the steering wheel from dragging us to the right. 2 hours later they picked us and the car up and took us home. Sorry for the long story but whats troubling me most is that this is deffinitely mechanical but i cant understand why it will do it so bad one minute and then drive fine when its had a bit of a break, anyone?