How can I fix coolant on 96 Passat?

We have an on going problem with a 96 Passat with only 53000 miles on it. It works fine for a while but gradually the coolant check light comes on more and more often. When we check the coolant is then empty. It does not come out the bottom or anywhere else we can see. Appartently it is getting into the oil some how because the engine blew and we had it and the water pump and several other parts replaced and now it is starting to do the same thing again. Does anybody have a clue what could be causing this problem?