How long after you add pH down, are you testing the water? This yoyo effect of up and down is very common when dealing with tap water. What happens is this:
*You're adding pH down.
*Now that your water is more acidic, the"carbonates" within the water, are able to dissolve.
*As carbonates dissolve, they act as an alkaline, and drives pH up.
*As the water becomes more alkaline, the carbonates become more stable, and the dissolution slows or stops.
This will keep happening until all carbonates are dissolved.
You can add a RO filter to your system, and removing the hardness (carbonates) from the water, before you mix.
You can wait longer after adding acid, before testing the pH, and reapply acid as needed.
If you choose the latter of the two, keep in mind that, the more you repeat the process, the less carbonates will be, and need to dissolve, and the more effect the acid will have in bringing down the pH. Basically, add less and less acid, otherwise you'll over correct.
Does this help?