Lotus pointed me to
Aquarium Chemistry, pH, gH, and kH relationship for something similar, this will surly help you out.
IMO as long as your not adding things to change the PH like chemicals (PH-UP,PH-DOWN,etc), or things like crushed coral/driftwood and your tap water/water changing water has a "like" PH, your PH shouldnt be moving that much. ... CO2 injection will also lower your PH
As the article points out, shifting PH is due to low KH, adding baking soda to your water will raise your KH (and your PH some too) and make those PH swings less and less till you get it dialed in.
I would only treat your water-changing water to raise the KH to make sure you dont over dose and wipe your tank from the PH/KH shift.
Also note, if your tank is cycling, your PH will be a roller coaster till it is done.