during veg i have noticed twice no where the ph has stopped going up and staying the same and a day or 2 later it lowers-
that's what it does...there are so many things that factor but you will get up's and downs not necessarily a problem but a pain in the ass...also are you growing with a rez if the water level changes then your Ph and PPM will to. just my thoughts!
i found a thread on GC that explained(very well) about the ph going up and down was linked to your plant drinking and the nutes ppm/ec level , i thought i had it saved but i can't find it now sorry but if i do i'll pm you it. Middy