That seems a bit high for DWC system. I think you want aroung 6.5 at the very highest but more like 5.8 or 6 is better. It needs to stay the same or very close at all times or you will shock/stress the plants.
There could be many things casuing it to flucyuate like that. I would use distilled water to start with and go from there and see if it still changes that much.
What kind of nutrients are you using?
PH is a tricky thing sometimes. I wish someone would post a clear howto properly adjust PH when first starting a fresh tank in a DWC or Bubbleponic system. Do you treat a gallon at a time while adding each gallon of water to your tank? When and how do you add the PH up or PH down basically. There seems to be more to it then just measuring PH, then adding up or down chemical, stir well until disolve ph up or ph down chemical is dissolved, then measuring ph again.
So if someone could post some details on PH adjusting that would be awsome!