I would not be surprised if you see pH swings up again if your nutes were designed to be used in soil. Often, soil nutes are buffered to come in in the high 6's. Hydro nutes buffer in the high 5's.
Another possible cause of the swings is not changing the res often enough for the weight of the total plant material that lives off it. The greater the plant:water ratio is, the more often you have to change the whole res- DUMP and refill- to prevent nutrient imbalances and the accompanying pH issues and lockouts.