Weird PH drift with low pressure aero set up

Hey all...
I'm doing a couple of trays of low-pressure/oakie aeroponic. This is the first time I've done this style for flower; I usually do flood & drain. I'm now about a month into flower and things are looking awesome. However, since day one my ph has been drifting consistently down.

Usually (with my flood&drain systems) my ph drifts up a bit until very late in flower. I bring my reservoirs to 5.8 (ish) every day, and usually I'll find that they've drifted up between .1-.3 by the next day (even in cases when they're drinking more water than nutes and my ec/ppm is climbing the PH will still drift up a bit). Again, this happens until the last 7-14 days of flower, when the trend reverses and PH usually starts drifting down in a similar fashion.

Now with this oakie aero system I see the exact opposite -- ph will drift from 5.8 down to 5.6 or even 5.5 day to day.

Has anyone else had something similiar happen? Any clue what's causing it? Anything I should worry about? Anything I can do to change this drift? I'm puzzled....

Thanks much

Hi guevera,
I've usually gotten a slow drift upward and I corrected down every couple of days (E&F). Are you using exactly the same nutrients as you did with your E&F? Some products can totally mess with ph. The other thought is root rot. Roots look OK? Using any enzyme or anti-slime regimen? What are you using to correct upward...standard K-bicarbonate Ph UP?
JD