S8 Braking problems - help!

hi everyone1
got a proper problem - my baby is diving around and shuddering when braking despite new front and rear tyres and discs/pads - Audi say they can't replicate the prob, it happens when I am braking down from 50ish thru to 40ish, get an ABS-type judder on the pedal and steering wheel shudders! no warning lights and this is in dry daylight conditions!
it's been suggested it may be the ABS rings and also the hub, don't want to spend much more trying to find out! anyone had similar please - she is a 52 4.2 auto with 69k
cheers in hope!!
Joanne