Ph Probe

This is for those who are running a computerised co2 setup. My PH is always
up and down, (ie the reading I get). The ph generally stays near to what it
is preset to but jumps either side of it. The waters ph is what I have the
controller set to.  There does not seem to be any thing wrong with the
equipment as when I place the probe in calibration fluid it reads perfectly
and does not jump around. The probe is placed in the tank where it recieves
average water flow. Any suggestions?