C3 citroen Braking Shudders

[EVILC]

when braking the car shudders. in morning i have no problems when i leave the house. but 40 miles later when i brake for the first time the brakes cause the car to shudder. it is like this happens after things warm up, because when cold and rested no sign of it. i have had 3 sets new discs in 24 months, each time things okay to start and slowly it gets worse. disc returned to manufacturer who report okay. change hubs and bearings still problems, any ideas? also noticed if i change down before braking things are a lot smoother.