I'm guessing top drip system, dripping into a DWC? Does the pump only turn on when the drip is running? Meaning, is the water in the bottom of the pots not circulating independently unless the pump turns on and increases the water level from the top, causing the water to move from the DWC back into the reservoir. When this was happening to me, PH and PPM would fluctuate whenever the drip pump would turn on. How frequently are you running the drip system? 15 minutes an hour while the light is on?