What else are you feeding? There are nutrients that will spike or drop your ph with ease. For instance, I use Dyna Gro Protekt....that right there is a ph up on its own. It will provide silicate and raise the shit out of your ph. Hygrozyme does the opposite, it will lower ph.....maybe one of your nutrients is doing it....check ph before you feed. I think i also read somewhere if you have root rot it will throw your ph off too....
Edit: just found the pages in the Bible

2 possibilities so far.....