Paul,
If adjusting the temp, and working with a larger volume doesn't remedy the situation, another thing to consider is moisture. Depending on the humidity of your working environment, frozen centers can collect condensation very quickly after removing them from the freezer. They can develop a thin layer of frost that can then melt into the warm chocolate, causing it to thicken and seize. Also, along with natural oils, freshly grated lemon zestcontains somemoisture. You can try using a drop or two of pure lemon oil instead of the zest.
I use Valrhona Ivoire, and it quite fluid. It's very hygroscopic though, and will absorb moisture from the air. I had anopen bag that Imisplaced, and when I found it and tried towork withthe remaining chocolate, it was very thick. You might want to temper a small batch without adding anything to it, and try dipping some room temp items, just to be sure it's not the chocolate that is the problem.