whats wrong with my abs?

i have a 02 mg zs. ocassionally when i apply the brakes they judder really badly, its like the brake is going on off on off really quickly and you can feel it under the pedal. it does it especially when its wet. i have recentlt had the cv joint on both sidews changed and had my breaks checked they are fine. as the abs light intermitently also comes on i gather its to do with that?