sometimes the sensors give a bit of trouble as the system in gereral is fairly reliable.
i had a similar problem as you discribe.i sorted by removing the sensors one by one from the hub assembly and cleaning the sensor face,but do it carefully as there is a magnet in the face.
next look down the hole the sensor goes in and you will see what looks like loads of teeth which are part of the cv joint,these are what the sensor uses to determine if the wheel has stoped rotating and thus engages the abs.
if all that fails carefully disconnect and reconnect each sensor cable at its disconnect plug (the front ones are in side the engine bay resting on the inner chassis leg.
failing all that at this point it may be worth getting a specialist to do a full diagnostic on the system or if you can afford it buy new sensors if at least to eliminate them as a possible cause