I also get the shimmy at 40mph but only when decellerating. Not real bad, but enough to be an annoyance. New tires, belt drive, total balance and alignment, check front wheel bearings, rear wheel bearings, front steering head bearings nothing was off and nothing cured it.
I think it may be the strange bushings used for the swingarms. They are a pubimeter larger than the axle hole and there is a very small amount of slop there. Every bike I have seen has been the same too. Jojje, the turbo Bonnie drag racing Swede, had the exact same problem and put tiny little shims in the swingarm bushings and it cured it. This winter I'll pull my swing arm axle and do the same. he wrapped once around with some sheet metal (sounds like a job for empty beer can to me) and put it all back together.
I keep my hands on the grips at the time too.
