I had quite a bad bump in mine at one point (not my fault!) and it did the exact same thing, light braking at around 50-60mph for me made it shudder the most. Heavy breaking or far above or below 60 mph and it didn't. There was another resinance point again at around 85mph for me. I replaced tires and brakes and in the end it was the steering arm. Have you knocked it up a curb or anything like that? Did it suddenly happen or has it increased over time?