There are a few things to consider to help improve the performance of your ORP readings:

1) The accuracy of a new electrode is +/- 20 mV. If you are seeing changes over time within 20 mV of a value, then it’s just noise. 

2) If the value is drifting up or down continuously, it’s likely due to the motion of the sample. If measuring an outdoor source, it’s best to take a beaker or cup to get a sample of the water and then measure it with the ORP sensor.

3) If the values you are getting are close to 0 mV, there may not be enough ionic activity to give a stable reading. Freshwater samples may need to have ionic strength adjuster added to them. 

4) Residue on the tip of an ORP electrode can cause the readings to be noisy. To clean the electrode, use a soft cloth to wipe a small amount of ethyl alcohol or isopropyl alcohol around the globe-shaped tip of the probe. Rinse the tip with distilled water, and repeat the alcohol cleaning one more time.