I ran into that same issue recently. Turns out its my water that is the problem. I have well water that normally has an EC of around 80 uS. However, it must also have a lot of carbonates in it because the PH shoots up wildly when ever I aerate or agitate the water.
I could take plain tap water that started at 6.7 PH, and aerate it for 15 minutes, and it would be close to 8.0. Just putting it in a jug and shaking it would raise the PH.
This wasnt so bad when I was running full strength nutes at around EC 1.3 or more, but when I tried cloning with low EC levels, it turned out to be almost impossible to keep the PH in the 5.8 range. In just a few hours the PH would shoot up to 6.7 or so - no matter how much PH down I used or how often I added it, the PH just kept going up within a few hours as long as I was aerating.
The other side effect was that it took a ton of PH down to lower the PH from the 6.7 range down to around 6.2. For my rez, typically about 60-80 ML of acid. BUT, adding just an additional 1 or 2 ml would drop the PH from 6.2 to 4. I had a heck of a time trying to keep it in the 5.8 range.
Like I said, higher nute levels were not nearly as much trouble. I assume the nutes were acting as a buffer. This was with a 15 gallon rez.
Maybe your water has similar issues…
It came up in this thread. There are some links to explain it in more detail.
By the way, I still dont have a solution.