I have been playing in hydro for while now. You are getting good information here. The slam dunk solution is to go to RO water but that also brings in other issues with your nutrients as it strips everything out of the water so you have to make sure you add the trace minerals back. If you do this cal-mag is a must to add back in.
I use straight tap as my source now as it is about as good as it gets where I am located. What I have noticed is they vary the buffing that they do at the plants at different times of the year so you have to keep on top of it.
In my source water and using GH PH down I generally use 12-16ml of PH down to 4 1/2 gallons of water. It instantly falls down into the 4's but with overnight agitation (small recirculating pump) it rebounds back up to 5.4-6.0. That seems to break the buffering. I tweak it from there and it usually stays put. I had issues like you are having until I took the time to get the PH stable before adding nutrients in and placing it in the feed bucket.
I use straight tap as my source now as it is about as good as it gets where I am located. What I have noticed is they vary the buffing that they do at the plants at different times of the year so you have to keep on top of it.
In my source water and using GH PH down I generally use 12-16ml of PH down to 4 1/2 gallons of water. It instantly falls down into the 4's but with overnight agitation (small recirculating pump) it rebounds back up to 5.4-6.0. That seems to break the buffering. I tweak it from there and it usually stays put. I had issues like you are having until I took the time to get the PH stable before adding nutrients in and placing it in the feed bucket.