Not to take away from further investigating your source water, but have you considered a coral bacterial infection? It's something not too many reefers list as a possibility, especially when all the parameters look stable enough, such as in your case. Exact causes are relatively unknown but the effects are well documented.
Joe Yaiullo and several researchers have discussed a doxycycline treatment as an option. It's usually a 24-hour treatment cycle and can be repeated after a week. I, myself, have gone through what you are experiencing now and have turned things around for the better after a 2-time doxycycline cycle. But, please note, this was based off my own experience from the advice of Joe Yaiullo, so the end result might be completely different for you.
Also, don't expect instant results, but rather a "lessened" occurrence of the RTN/STN episodes. Not to say that you'll never have a coral die after trying this option, but it is one more variable to eliminate. The plus side of it is that there is very little, if any, side effect of dosing directly into the display tank. So therefore, why not try it. Might be worth a shot, before you start messing with the kitchen sink drains.
Attached is the exact instruction I was given direct from Joe, himself.