Puff Monkey,
It might help if you posted what variety you are growing, what nutes and additives you are using and what kind of hydro you are running. You probably have posted this befoe but please refresh our minds.
I have never grown hydro yet so I'm trying to remember what I read about PH in the Forums. If I remember correctly, some nutes seem to cause wide swings. I have read that some brands of nute have "buffers included" to keep the PH stable. If so, what MIGHT be happening is that your nute have buffers in them and when you PH the solution you over power the buffer(s) But after a period of time the slow release buffered over power your PH down, so the PH goes up. If this is what is happening in your case, then you are kind of screwed with a choice between playing chase the PH with the nutes or find other nutes that doesn't do that.
Incidentally there are some excellent threads on both the PH problem and making your own PH down on the forum.
I hope I'm all wet with my assessment of what is happening with your PH.
How long does it take your PH to go up, how far does it go if you leave it alone, and will it stabilize or even come back down if you don't mess with it?
Today is probably not a good day to get answers on the Forum being a holiday so hopefully you will get some bettr advice tomorrow.
Good smoking.