Re: Suspension setup
Well it doesn't happen on my Blade, but then I have a different model.
Maybe your 'certain speeds' are higher than mine though!?!?
Have you tried riding in to the wind and with the wind, to see if the speed at which it occurs changes? If it doesn't, then I would guess that it's not wind resistance causing the vibration.
Does it have a non-standard screen on it? If so and you have the original, might be worth swapping over to see if it makes any difference.
Other than that, I would also say check the tyre pressures (dunno if it will help but always worth checking).
Or it could be warped discs, or poor wheel balancing perhaps? (Warped discs being the worst case scenario, they could cause it to fail the MOT). Does it vibrate under braking? This can be a sign of warped discs, but I believe if they are only slightly warped you might not notice anything.