May I share my experience... I have a 03 dII and I was using roughly 1/2 gallon of coolant every few days similar to your situation, I figured it was a head gasket as well until I talked with several people here and also some indy mechanics... I crawled under the vehicle to find after driving it a short distance to warm everything up there was coolant dripping down from the top of the bell housing onto the crossmember bar... if you are seeing symptoms such as what I described, I would think it could be a lower intake manifold gasket... I fixed mine myself and have had no coolant loss what-so-ever.