I'm pretty new to this too, so don't take my word as gospel. But, after encountering the same problem before, the first thing I would ask you is what kind of water are you using?
Problem I had the most with ph was with tap water.
Municipal water supplies are often treated with some pretty potent buffers which will try to neutralize the ph if a foriegn matter is introduced somewhere in the system. That's why it "rebounds" like that. And it will continue to do that until the buffer is exhausted.
I started using bottled water instead, because I don't have acces to ro water, which is supposed to be the best. I've found that distilled water is about the easiest to adjust, and cheap!