I had a constantly rising ph a while back, it drove me insane, I was constantly checking it, adding drops of diluted ph- when it rose slightly. I was setting it to 5.8 and within hours it would be 6.1
I did what THG suggested, raised the nute concentration and set the ph lower.
You see, as the plants use the nutes, the ph will rise. So, to combat this, you have a few options:
1. Raise the level of nutes. This means there are more nutes available for the plants to chew on before it affects the ph.
2. Use a bigger res. The bigger the res, the more of a buffer your plants have, again before their feeding affects the ph.
3. Stop checking the ph. Now, the way I understand it, most nutes have "buffers" in them to keep the ph within the target range, let them do their work. I found that when I left mine alone and stopped checking, they stayed at 5.9/6.0. Higher than I would like but its not what I would like, its what the plants like that matters, they seemed happy, were growing much better than when I was constantly fiddling with them and giving them ph fluctuation to deal with, which can cause problems of its own.
4. Have a lower starting ph. I set mine at 5.8, so I was actually causing lockout of some nutes, I now set it to 5.5 as THG said and even though it does fluctuate a little, it settles in the 5.8/5.9 range now (AN nutes)
This is not exactly fact, it is my experience or similar problems. I listened to the guys here and it helped enormously.
Green mojo on its way.