IIRC back in the late 90's, one of the bike mags ( Bike possibly?? ) did some research into this phenomenon on a series of bikes.
The common factor was that many of them did this head shake when you rode hands free - it happened anywhere between 40 & 60mph and went away as soon as you took hold with one or both hands.
The answer they came up with to the problem was to spend time setting up the suspension correctly to suit the rider - as soon as this this was done, it virtually eliminated the problem.
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