Well you have been very helpful. You allowed me to maintain my sanity during this. So for the rest of the story...
I recently received two Haku Xeta's. Both are anodized titanium. One is blue and the other is gunmetal.
The blue one is the one that I was having trouble with. So I built the gunmetal one. Before doing so, I sanded off the anodizing on the clamps with an emory board, based on your comments about making sure that the connections were clean. I also disassembled the center pin which holds the positive clamp assembly and made sure the threads were clean. I then installed the coil that I had used in my first attempt with the blue Xeta. It is now working just fine with temp control.
I also have a raw titanium version and that one never had a problem. I suspect it was the anodizing on the clamps.
Rather than drain the boro tank and rebuild the blue one I am going to use it in Power mode until I empty the tank then I'll give it the same treatment.
The Xeta is a relatively new atty for the Billet Box boro so this may come up again as it gets more popular. I don't often come across Billet Box users that use temp control so perhaps not. It's a shame because the DNA60 is such a great little chip for temp control.
Thanks again for your help. For now I think this one may be solved.