I had been using EBC reds, got the shuddering, and then switched to the yellows. I was very careful to bed the pads according to EBCs instructions, but on the second lap of my first session the shudder returned. If I backed off for a lap to cool the brakes, they felt fine. Start using the brakes hard again, and the shudder started back up. It was so bad that I couldn't try to ignore it; it felt like I was going to break something if I persisted with it vibrating that badly.
I've got many years of racing experience, and have bedded new brakes many, many times, so I'm confident that I'm not doing it wrong. At one time I raced a first generation RX7 that was very fast in a straight line, but the car had really small brakes for it's performance, so I learned to manage the brakes. Our races were half an hour long, so if you managed things correctly, you'd have brakes for the whole race.
If you tried to outbrake another car, then the brakes were done. The best brakes I found were the Porterfield RS4's - the fronts were good for one race, the rears were good for two races. So it's really frustrating for me that I have a car with relatively big brakes that is quite a bit slower than the old RX7, but I haven't been able to use those brakes.