Issues with pH sensor

Hi! 

I've been having issues with the Analog pH meter Pro. (Gravity: Analog Industrial pH Sensor for Arduino - DFRobot)

I've been using an ESP32 and one of the ADC inputs (GPIO36) to drive the pH meter. The issue I'm getting is that I've been trying to create the linear slope using buffer solutions, and to do that I've been getting raw readings from the probe. The issue I'm getting is mostly that I have get a value of maybe 2700 for pH 7, but then, all of a sudden the value just jumps to 4095 and stays there. I touch the pH meter  v.1.1 and the voltage drops down, but then it goes up to 4095 (the max value that ESP32 outputs) again. After I've disconnected and connected a couple of times it goes back to the right value, but then it suddenly jumps again. I've found that if the module is disconnected I of course get 0 volts as a value, but if the probe is disconnected I get 4095. What could this mean? Could it be electrical disturbance somehow, or is there a problem with my module?

If you want any further information just ask and I'll gladly get it for you.

I'm grateful for any help!