NO baking soda! (sodium bicarb) Sodium bad!
Use pH up (potassium hydroxide). Potassium good!

Do you have untreated hydroton or RW in your system? that's usually the culprit, and it's not anything to get overly worried about.
Add your pH down slowly and set to 5.8; if it swings back up just set it back down. It should stabilize and require less fidgeting once the system and media equilibrate.