How to acurately measure pH in heterogeneous solution?
The $\mathrm{pK}_{a3}$ of phosphoric acid is 12.319 according to Wikipedia. So at a pH of 11 most of the phosphate species would be in the form of $\ce{HPO4^{2-}}$. Thus as $\ce{Ca3(PO4)2}$ precipitates the solution will become more acidic. So an unanswered question here is how fast would equilibrium be established in the solution. Normally once precipitation starts, then equilibrium is established fairly quickly. 

Thus it seems likely that equilibrium would depend a lot on how well the solution was stirred, thus swirling the ppt in the solution rather than having it sit at the bottom...

Also the solution is basic enough that $\ce{CO2}$ from the atmosphere will be forming carbonate species which would also slowly make the solution more acidic. (not sure how much drift in pH that you are observing...).