As I understand it, rinsing in distilled water is best instead of tap water. Could the battery be getting low?
I have heard that I should be using distilled for rinsing, I'll change that part of my process. I looked into the battery aspect but according to the instructions it has a low battery indicator which I have not seen light up. Also according to the instructions: "The meter is also provided with BEPS (Battery Error Prevention System), which avoids any erroneous readings due to low battery level by automatically switching the meter off."