I have had this problem with my '63 +4 since I bought it a year or so ago. I checked all the usual culprits and replaced the damper blades with no luck. Yesterday I received a kit from Mulberry Fabrications in the UK.
http://www.mulfab.co.uk/ which provides all the parts to install a steering damper.
Installation took about an hour and is fairly straight forward, involving drilling one hole. During a 15 mile test drive down all the bumpy roads that I could find, which normally induce the dreaded shake, I experienced none of the usual symptoms. Fingers crossed that this permanently solves the problem.
