Part of my Dino eradication process was increasing my nutrients. My rock and substrate became PO4 saturated. My PO4 got up to .28ppm. Once my Dino's where gone I started the lanthanum drip treatment and it took 3.5mo to become stable at .08 which is within my desired PO4 range. I was seeing the exact same thing you are seeing with PO4 coming down and then going back up. What I saw was after the first month of doing the drip my PO4 started coming down slowly. Every time I dosed it would go down and then go back up to .28 for the first month. Then the next 2.5mo I saw the following: .28 to .24, .26 to .24, .24 to .21 then .23 to .18, .20 to .16, .18 to .15 and so on until I hit my goal of .07-.1ppm. There where many PO4 test between the ones I gave but just wanted to give you an idea on what I saw. IMO just keep up with the dosing and you should eventually get to your goal.