Well, without knowing all the details, I would take it back to the basics:
Did you check your sump for any contaminants (metals, batteries (tossed in by a kid), rusting impellars)?
Did you check for stray voltage?
Did you verify that your tank's temperature is accurate with multiple devices?
Did you verify that your salinity is actually what you think it is, testing it both with other equipment as well as to have someone else confirm those readings?
Did you have someone else test your water with their kits to verify that your numbers are accurate?
Did you check your timers for excessive lighting, excessive heating, or excessive cooling periods?
Did you test your RO/DI water for Ammonia, Phosphate, Nitrate, and TDS? Did you test for chorine?
Did you test your newly mixed saltwater for everything, including Alk, pH, Cal, Mg, Ammonia, & Salinity?
Did you verify that all equipment is working correctly, including calcium & kalk reactors, making sure the media looks right and the pumps are functioning as they should?
Did you disturb your sandbed recently, unearthing anoxic zones?
Did you remove LR from your tank for a duration, exposing sponges to air which could die off and cause issues?
Did you have Xenia die-off en-mass? Flatworms?
Did you treat with anything lately?
Do you dose anything?
Is your tank grounded? Is it still grounded, or did the probe come loose?
HTH