It only shudders above 55 to 60. Light or hard braking makes little difference. Its all about heat. The high speed of the rotor means the sliiigggghhhtt bit of unburned deposits on the pads and rotors can trigger their trademark vibration, which the steering wheel picks up on and amplifies (badly). Brake lightly and longer, or brake hard and sudden.. and you'll generate enough heat in the pads and rotors that the deposits are no longer 'chuddering' past and now 'flow' with the braking action. Vibration goes away after 4 or 5 seconds. Drive for a minute and cool the parts... the whole thing starts over again when braking again.

The reason new pads and or rotors appears to fix the problem is.. they have no deposits on them yet. It takes a few thousand miles. In other words.. I have literally.. removed the pads after the shudder showed up, sanded them lightly on the belt sander, reinstalled, and the shudder was gone for 3,000 miles. Then.. eventually.. it comes back. Rather than go back to high dust pads I'm seriously considering just prying the pads back a bit once a year and running some sand paper on them. I have to swap the snow wheels out twice a year anyhoo, so what the..