One potential cause for the changing pH over time is that proper use of a pH meter requires a gentle stirring motion. This is because the gel inside the pH meter is gradually seeping through the porous glass bulb at the end of the electrode. Without stirring it will cause a localized imbalance and you'll get a false reading.
So, for example, if I take a wort reading while I am gently stirring the electrode in cooled wort, pH meter stabilizes and I write down that value. Then if I was to simply leave the electrode sitting in that same wort - the value will drift over time. By stirring again, it should come back to the initial reading (repeatable measure). If it does not, then there is more going on. As I read the initial post, just sounds like incorrect method.
This thread has been a good read for me, thanks!