I stir after each bit of fert or additive good and proper and PH lastly, but i presume you do that. I mix up 80 litres a time and if i do check PH for some reason or another and its out i give a real good stir and after doing that it seems to be about right again give or take a wee bit. Tbh i cant remember last time i checked it for no reason at all it would do my nut in faffing about all time for no reason, basically if plant looks good i let it get on with its stuff once I've set the PH and i dont check it again unless theirs a problem, really i cant see point in doing that.
I mix up 80 litres a time and take a 10ltr bucket full each time, but before filling bucket i always stir my tank. Don`t know why but using air-stones can cause big PH swings as you've found out. Also running a bigger tank or rezz full is gonna be much more stable than and running a little tank full or i should say it stays stable for longer, i dont know why that is either but it does Hmr.