It's highly likely the lactic acid bacteria in your starter (these were living on the wheat in the field, some maybe from your hands) are more active than the yeasts. You need to dilute the acid out of the starter, feed it more often and keep the whole shebang above 16°C, preferably 18-20°C (although you can still store it in the fridge). Low acidity, more feeding and 18-20°C will raise the activity level of the yeasts relative to the bacteria and generally keep the yeast enzymes, bacteria enzymes and wheat enzymes in harmony.
Consider Ed Wood's "washing" process, it's a very effective starter management procedure. It's going to be a bit different for a mother dough method but the ideas are still the same. I've written about this here before but for somewhat different questions.
Don't pour off the hooch. This stuff is the dark liquid that forms on top of a starter when dormant in the fridge. It is protective (acids, alcohols and enzymes) for you and the microflora, and helps the microflora digest food. It actually increases starter activity. Ed Wood says the starter is contaminated if the hooch forms in the middle or the bottom.
Don't worry about weighing exact amounts of flour and water when you're feeding it. Just focus on the sourness and the consistency. A thicker consistency gives a home cook more flexibility with time and it's easier to gauge the activity level.
Molds are really quite slow compared to yeast and bacteria, especially with hooch retention. The starter has to sit there for a long time to become moldy.
Ed Wood's Washing Process
- Stir hooch into starter.
- Increase the volume 3-5 times with tepid water
- Stir until homogeneous
- Pour off 4/5ths of it
- Tip in flour and stir until homogeneous
- If not a consistency you like then add more flour or water and stir.
- Wait until it has risen double or triple, don't worry about how long it takes.
- Repeat several times if it still tastes sour.