Stop using vinegar and get some Ph up and Ph down from a hydro purveyor....vinegar doesn't work well as you have found.

willard3 wrote:
....vinegar doesn't work well . .

Vinegar works very well willard. It forms a low pH-buffer with the calcium in the nutrient which tends to stabilise the pH.
kea has a problem of a different kind. Perhaps the medium contains calcite (shells, limestone) elements or that she might have an infection, in which case she should take the container apart to see if there are any rotting parts around. After that . . . clean and replace, and start again.

I don't think it's the medium. I'm using the same medium in various other pots without this problem. I'm guessing something's infected. There was a bunch of dead roots, so I cut them off as best I could, flushed with a lot of water, and re-potted it. If the pH keeps going wonky, I'll have to break out the bleach. (Yeah, I'll take the plant out first, I'm not that thick.)

You can bleach the plant as well. It's recommended. Make a very weak bleach solution. So weak that you can just smell the chlorine. Stick the roots in there for ± 15 minutes. Shake the roots around, then take the plant out and put it in clean tapwater.

The problem recurred again after 2 days, so I washed out pots, pellets and plant in dilute bleach. Looks like the root damage is quite extensive. I'll be keeping this plant indoors for a while and hope it manages to grow some more roots. And if it doesn't, at least I can eat it.

Chlorine bleach is difficult to control and quite residual. If you want to sterilize your system, hydrogen peroxide (1000 ppm) is much less residual and does the same thing.