Read through page 2. His phosphate was only at 0.16 the whole time.I don’t think it is unstable. I think you just have high phosphates, probably stable around 1.0 ppm. You are dosing too much lanthanum and plummeting the PO4. Then PO4 leaches out of the rocks to re-stabilize at 1.0 ppm. If you continue with lanthanum, do it slowly as @Miami Reef suggested and over a long time. Eventually it will come down, and your corals will be happier with the gradual decrease.
Decimal points are very important.