Do you mean that it comes on at odd times when driving or sometimes it just doesn't go off from starting up?
I have no VW experience, but abs generally checks itself after start up.
If the light doesn't go off from start up, it suggests the control unit or its wiring has a fault.
If it comes on after driving for a while it suggests a problem with either the sensors on the suspension, the rotors on the hubs or a connector plug on the sensor wiring.
There may also be a fuse/pressure pump problem.
I would go with Doug on this.Get the experts to check it.
It may be something simple. like the sensor lead to connector on strut/suspension mount which you could look at yourself..Any signs of chaffing/broken wire/dirty connection will cause it..But if all looks fine.Best to get it checked out.