Good morning DW1...DWC ph issues are really frustrating. I know...maybe a few tips will bring the ph back in line ....
1. cool your reservoir....add frozen bottles of water....refreeze and replace when they thaw.
2. cool your reservoir...put your air pump in a small fridge (you have a fridge for seed storage and to keep your choice of beverage cold right? ) .
3. change the reservoir water weekly, after the plants have taken the nutrients out of solution...the ph will start to edge upwards
4.make sure your ph tester is accurate...my nutra-drip tri-meter won't give an accurate reading when my HPS light is on.
I'm sure others will chime in with more tips, but this should get you started....enjoy!!