You could be right Mr E, my water is well water so there is probably a big difference between that and tap water....you could add an entire bottle of ph down and it would only hold it for a day or two before crawling back up....
I do find that when I first do a res change it takes, as you say, very little but does not stay very stable for the first day or so until I have added more ph down etc, then it seems to stay pretty much where I put it about 48 hours in. Now flowering is a different story with the ph probs

I use baking soda as ph up usually as I only have to use small amounts and it's easier than ph up for me....I doubt I would use it if I had to raise the ph a few points or so....or have to use it often but I have not had any problems using it so far....keep it on hand with my other crowd of bottles. I have found it raises ph kind of slowly over a few minutes so I add a little and then check it a few mins later because the number seems to rise a little more after a min or two. I don't seem to have that particular situation when using ph up/down so just a a heads up.