It's your hydroton.
Keep adding your pH down until you get it stable over the course of several hours, then do a COMPLETE res change and re-fill with whatever your water of choice is- tap water if it is not too 'hard', or distilled water with calmag in it. Bring that to the correct pH and you should have a much more stable pH over time.

Your problem is that you MUST soak hydroton in water pH'd to 5.8-6.0 before use, and let it sit at least 24h to determine that indeed it IS stable, then rinse well and use.