Have you hooked it on vag-com?, don't be foolish enough to start spalshing money on things you may not need. It may even be a faulty sensor or wiring to the sensor.
I've had this problem before, intermittent abs light glowing on n off. Showed up as sensor faulty, bought one, put it in, still the same, so checked wiring and the wire that goes into clip had broke. Try the easy route first before buying expensive/un-wanted parts mate.