They don't last for ever. My first probe didn't last long, but wasn't giving erratic readings, instead it just continued to drift.
For readings to be practical and meaningful they should be taken with the sample and probe at very similar temperatures, close to 20C and constant. For our purpose perfect precision isn't essential, but we should avoid taking potentially misleading readings. The following is my procedure. The probe is rinsed in DI water before and after every stage and excess surplus DI water gently shaken off the probe.
With DI water, probe and sample at the same and right temperature, the sample is gently stirred using the probe. The probe is left in the sample and allowed to settle while the reading observed. This is repeated until the reading stabilises quicky and remain constant for about a minute, then recorded. The rate at which this happens depends upon the buffering power of the medium being measured. Buffers used to calibrate the meter will settle to a constant reading more quickly than will, for example, low gravity wort runnings from a thin mash created using RO liquor. DI water itself has virtually no buffering power and therefore a large range of potential pH.
My first probe was replaced when the pH reading began stabilising, but continued to rise very slowly for many minutes. I replaced it with one of
these, although maybe not this particular supplier. I found this better than the original in that it has lasted much longer and used for vastly more readings. It also can be opened to allow the calibration fluid to flow into the membrane and be topped up using the syringe supplied, something I was unable to do with the original.
As your readings were erratic Guy, I wonder if there might be an intermittent contact in the wiring. Also, as no doubt you have found, any reading can be disturbed if the body is moved when taking a reading. You can get pH probe cleaning fluid, but it will likely cost more than the probe above.
Without patience, life becomes difficult and the sooner it's finished, the better.