This is very similar to the problem I was having. I hate using additives, but I finally broke down and did. I discovered that the Kent Superbuffer did nothing for me. I started using the buffer made by seachem and after one use the PH was good for a few weeks. Then I added it once more and I have been staying steady now for a 4 to 6 weeks. I also drip the limewater.

Are you still having the problem? Have you noticed coraline algae flaking off or degression in coral growth?

From what I have seen in my tank, things will start to look worse within a few days if there is a problem with the PH and Ca. :!: