we used to get a 'coherance' fault on Peugeots on the front and it felt like the abs kicked in at slow speeds when the abs ring was cracked, it saw the crack as a longer gap between teeth and a lock up, hence kicking the abs in for a short period
or
it would also appear when the rear sensor spacing was too far away from the magnetic ring, reseating the sensor worked a treat by removing the corrosion/flakey rust under the sensors plastic body, and sometimes we would add a m6 copper washer between sensor and hub to cam the sensor in, when done right it would just clean the accumulated metal filings from the brakes from the magnetic ring