hmm, well that knocks out the two most common and obvious causes - drought and being potbound! If you want to respond underneath your question, you just click in the box as if you're answering your own question, Marionbay, in the same way as I've done here.
Is there any chance you could post a photograph of the tree? You can either add it to your question or post it separately again. I seem to recall you said there was healthy growth near the base, but that the top was dying back...