If you recirculate, you need a larger res, even with lower wattage range like 250's each plant in a dwc gets a minimum of 5 gallons to start with. Even a smaller grow like that is challenging to maintain stable PH/PPM in late flower. Since you are growing outside, I'll say your container size would be even bigger.
My only concern is how high your PH gets, 7.5 is literally tap water. What strength mix are your nutes.. ?. And whats your water quality like?.
Troubleshooting suggestions
1) Try a larger container, Its harder for the plant to change the water chemistry if there is more solution..
2) Check tap water quality, IF it sucks try using RO water, its a bit acidic, and helps keep PH down if thats your goal.
3) Higher concentration of nutrients, You did not provide any PPM data, Nutrient PH buffers work better in higher concentrations.
4) PH down, vs using nutrients to keep it down. Mix to proper strength, add PH down.
More or less generalized info because I have no experience with Dynagrow.
Hope this helps.