I would like to address how the nutrient solution is being prepared.
What process are you using to mix everything up WNL. How long are you allowing it to rest?
I find that pH fluctuations are normal and should be expected and anticipated. To alleviate large swings in pH it is important to allow your nutrient solution time to rest before using it on your plants. To prepare my own solution I will mix my A+B solutions in water and allow them to rest for 1 hour before adding the rest of the nutrients. I would then allow this solution to balance for another 30 minutes. Then pH to the right level. Then rest for 1-2 hours. Then pH again. Then rest overnight, or 8 to 12 hours. Then pH balance one last time.
By the third balancing in 12 hours I find my nutrient solution will not deviate more than 0.1pH in a day when resting in the reservoir. When applied to the plants you need to be aware that the removal of elements from the solution as the plant consumes them is going to alter the pH, so you will always need to be very mindful of your pH in hydroponics. I wouldn't even think about hydro without a pH meter, much less a constant monitoring.
Nitric Acid pH down from Technaflora holds pH much more stable, but it has a greater impact on my measured EC than GH phosphoric acid pH down. As a result, I reduce my projected N ratio in relation to PK and Ca levels in order to compensate for the nitrates and nitrites provided by the acid.
Some systems are pH balanced and stable when you run all the stuff and you have reasonably clean water to begin with. Having a better nutrient system could improve your stability. Consider Botanicare CNS 17 Hydro, GH FloraDuo, or AN Sensi A+B pH perfect.