Take all your wheels off and clean the ABS sensors behind your brakes. If this does not cure you problem - do a bleed of the attessa system. It is quick and easy and will only cost you a few bucks. If, by chance, the problem still exists, get your mechanic (or someone who knows these cars) to check out your attessa pump. More than likely these fail and cause a surging through the car at cruise speeds, not while cutting sik so this is a last resort to check as it is in a difficult to get to position above the diff centre. You might also wanna check your attessa reservoir in the boot and make sure it is topped up.