All,
I just wanted to update everyone on my grow. It's been 3 weeks since my last post, and I haven't seen any rapid pH swings since. To recap, I went from 5.8 to 3.2 overnight, 2 days in a row. Now, I do see a slowly decreasing pH over a span of 3-4 days, going from 5.8 to 5.5. Usually by then, I need to add water to my res, so I top it off with a gallon of 6.5 water with nutes and that brings it right back to 5.8. I've continued with the recommended GH Flora nute mix for flowering, which gives me about 900-1000 ppm for the nutes.
I do see quicker swings immediately after a res changed for the first day, which Ive used a pinch or 2 of pH Up to get back to 5.8. Once that initial variation is taken care of, it then follows the slow decline of pH I mentioned above. As I wrote above, it appears that Woodys hypothesis is correct.