Interesting point, Quarryman. I first noticed this 'problem' some years back. I'd get a 'wobble' at around 40, mostly noticed when rolling down on relaxed throttle. Took it to A. Jefferies, but they found nowt wrong, did suggest the tyres (Tourance). I had the tyre removed and re-fitted, plus wheel balanced, but no improvement. In the end I just decided to ignore it, unless I was 'playing', and letting go of the bars as I slowed, to see if it would develop inh to a good old-fashined 'tank-slapper'. Never got anywhere near, the wobble had gone by the time I 'd dropped to 25.
I've got Conti Trail Attacks on now; I'll have to give it a go when I eventually get back on the road, just out of curiosity...
Cheers,
Dave.