Which part of the sump? Do you dose in the same area as the probe? any powerheads near the probe? How's the cable run to the apex unit, near any power cables? Apex Classic or 2016?
I get major PH dips when I dose in the same chamber. Usually temporary, but it will look like a poltergeist attacked the Apex. Also had issues with stray voltage from a heater that was on its way out. Numbers went weird when it kicked on.
Edit: you may have some issues, there is no daily PH swing @ all! The photo is my setup today.
Yes it's been calibrated. It's by a heater. Never dosed by it. It will work fine stay in low 8s then drop instantly or jump up instantly. My tank has a ground prob in the overflow and one in the sump