You're doing everything right. I've had a few that were like that; can you check your roots? The buffers in the nutrients should be able to control the swing. You can run the gambit and start checking the separate run off from each plant. That's what I do first when this happens to me. There could be some slim somewhere in the grow medium for example. You're not using rock wool are you? The only time I had that problem was when there was one plant that had slim on the roots around the ball so the long roots did not show. The only other time was when I was still using rock wool.