There are two potential causes for this issue.
1. The lead from the sensor to the circuit board in the O2EII is damaged and giving an intermittent fault
2. The sensor is depleted or faulty
If you have a multimeter available, you can check the voltage across the O2 sensor as shown below.
The voltage should read in the region of 50mV if it is working correctly.
If the sensor is not reading correctly, then it is likely that the oxygen cell has be must be replaced using the steps outlined here.
If the cell reads correctly, then it is likely that there is a problem with the lead from the sensor to the circuit board, to solve this issue you it will be necessary to contact an analox certified technician. You find details on how to contact Analox by visiting our website at https://www.analoxsensortechnology.com/contact.html .