The top things that can cause these issues include: sample homogenization, dirty probes, probe setup, probe selection, and calibration practices.
Is your sample homogenized?
Stirring your sample helps to keep your sample homogenized. This can help to stabilize your EC readings.
Is your probe dirty?
Depending on your sample, debris could be liable to stick to the probe. Clean the probe regularly to prevent buildup between the electrodes (two electrode probe) or blockage of the vent hole (four ring probes). Rinse the probe in between readings, and before/after storage in order to help prevent buildup. Especially when working with water, residue may not be visible to the naked eye, so when in doubt, rinse.
Is the probe set up correctly?
Make sure that the probe connector is properly connected to the meter (if using a probe with a cable). For 3.5 mm connectors, ensure that the pin is inserted all the way into the port. For DIN/Quick DIN connectors, double check that the pins are lined up properly.
If your probe is a new four ring probe, take a look inside the probe sleeve near the bottom. There is a small clear rubber circle around the internal section of the probe. This is to help preserve the probe during shipping, but it can interfere with your readings. Carefully remove the rubber circle, then re-calibrate the probe.
Did you choose the right probe for your sample?
Check that your probe is correctly submerged. Two electrode probes only need to be slightly submerged to cover the plates. Four ring probes require that the vent hole above the rings be submerged as well. Make sure that there is at least one inch of clearance between the probe and all sides of the beaker.
Check the usable measurement range for your two electrode probe. If your sample is out of range you need a two electrode probe suited for that range or a four ring probe with an extended range.
Did you calibrate?
Proper and frequent calibration is key. If the probe is used daily, calibrate daily. If not, calibrate the probe prior to use.