I would be looking over the suspension again very carefully.
You may have replaced all your bushes but one of your control arms may have a small crack or slight bend in it that would cause it to flex or bend under heavy braking resulting severe bumsteer condition.
Other issue could be your flexible brake line could have collapsed which you will never see from the outside, so replace them with braided ones see how you go.
If it all fails bring it to my work and i will go over it and give my advice free of charge.