I will bet money it's your RODI... when was the last time you changed your filters?
I'll try to make this short, but your situation sounds very similar to a situation I was having with my parents' tank (I do the maintenance on it, and it's identical to my system at home).
Things would be running great, and then out of the blue things would go down hill fast. I was chasing everything... blaming bulbs, rocks leaching phosphates, looking for stray current, chasing parameters... everything I could think of. Of course I resorted to more frequent water changes trying to get everything to stabilize, but things would steadily get worse.
Over the course of 5 years, this happened about twice per year, but I could always get things going again. The problem was that I was changing so many things at once, that I could never zero-in what the actual problem was. And, it was pointless to even try to keep SPS. We would add them while things were looking good, and then the decline would begin again and cover everything with cyano a few months later. The only thing that did well were the anemone, the fish, and a few easy to keep corals.
Even though I was always producing 000 TDS water, I eventually became skeptical of the RODI (started seeing a pattern of things getting better about 2 months after a filter change). It turns out that's exactly what the problem was... specifically the carbon filter. I went from changing the carbon filter every 6 months to every 3 months, and things have been great for about 2 years now (knock on wood). I could go into even more detail here, but something in my parents' local water was getting past their RODI even though they were always producing 000 TDS water.
