hi there...
i also have a is200 sport (99)
ive had the same for the last few years.. had around 5 or 6 different disc,s and pad set,s.... now ive got ebc disc and green stuff pad,s but still the same. now iam thinking of changing the whole system. but what you said about the ball is making me think. but i have a full lexus service history, so shorly they must of picked up on it..
would like to fix it if there are any more ideas please get in contact..