1995 Ford Windstar

Several months ago my car radiator clogged up the car overheated and despite stopping immediately the head gasket was corroded and was replaced (along with the radiator) -- since then the car has not been the same! 4 times I have driven the car away from the lot everything being fine and the car driving like new -- after about 2 weeks it begins to drive rough, to overheat, and with white smoke from exhaust-- diagnosi-- blown head gasket again with corrosion in the cylinder head -- the car has been fixed each time at a specialist in cylinder head rebuilding who has said he has never seen anything like my car that fails after 2 weeks. The Ford dealer says he ha checked everything out each time adn there is no other problem and in fact the car drives great for about 2 weeks -- is there something special about rebuilding a windstar cylinder head that the regular aluminum doesn't work? Is there something that they are missing -- why does my car only last for 2 weeks?