Chevrolet Metro lsi 1.3L 2000

Can anyone help? I have the above vehicle which i have a problem when i brake from about 45/55mph. I get vibration in the brake pedal but at slower speeds the brakes work smooth no vibration at all. I have put on new front brakes and shoes new front brake rotors, re torqued the wheel nuts and the problem is still there. Has anyone else had this problem if so whats the answer please.