This may be no help at all but last year I had a severe periodic vibration that was unnoticable under about 50mph and bad enough to break the silencer mount over 75mph.
It came and went regularly over a 3 - 4 sec cycle. It turned out to be that both front and back wheels were out of balance (by quite a bit - unbalanced rim clamps) and the vibration from each was going in and out of phase cancelling each other out at some times and building up to a "super vibration" at others because the front and rear wheels were different sizes (21 front / 18 rear) and revolving at different rates.
The obvious fix was to balance both wheels and I've had no problem since.
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