Are you adjusting before or after nutes? If after good, are you letting the rez sit for about an hour or so before checking and adjusting PH? I had the exact same problem, I would start with water, add nutes, measure PH, and adjust. Next day low PH. After some research I found out letting the rez settle before adjusting made a world of difference. I was checking my PH immediately getting, this is a made up number, say 7.3, I'd adjust to close to 5.8 next day like you in the 4's. After letting the rez settle my PH was actually much lower. I was using way to much down. I went so crazy I now have 3 PH pens. I blamed it on the pens giving me bad readings. I honestly have about $150 in pens. Lol. Hope that helps.