Hi Katherine,
I remember reading Randy Holmes-Farley's article on the relationship of Calcium and Alk...
In the section labelled "Apparent Excess Demand for Alkalinity" he says,
"If an aquarist is supplying a balanced additive to his aquarium, and calcium seems stable but alkalinity is declining, it may very well be that what is needed is more of the balanced additive, not just alkalinity. "
Perhaps your tweaking with the buffer is becoming counter-productive. Try doing some water changes and get back on a balance dosing schedule..or if you are on a reactor..try adjusting the dosage.
You can read his article here:
http://www.reefkeeping.com/issues/2004-12/rhf/index.php
Hth