Hi Hopsta,
Great discussion. I think a through a process of elimination you can sort this out:
- If not already done replace everything that touches cooled wort (eg no chill cubes / fermenter / hoses / etc) to ensure you don't have any bugs living in (possibly invisible to the eye) scratches
- Get water from a different source, pass through a carbon filter and adjust with bru'n water based on the water report. If you use distilled water go with 0 for your initial water estimates
- Ensure you are managing your yeast properly (it sounds like you are an experienced brewer, maybe you can already rule this out)
If you do both of these and still have the same problem maybe you need to be looking at your process, equipment and ingredients. A pH meter is a nice thing to have if it is reasonable quality to be able to rely on it, it is calibrated and you use it properly... if not you might be leading yourself astray. I would suggest that if you use bru'n water properly with the water report for local water or distilled water you should be close enough that pH will be within acceptable limits.