If you're getting extra aeration, then the pH will go up due to carbon dioxide off gassing. Then, the acid pump turns on and the pH and TA go down. This is the basis of the lowering TA (Total Alkalinity) procedure.
Then when you add bicarb to raise the TA, the process starts all over. It's an endless cycle.
The way to break the cycle is to maintain a lower TA (50 to 60)
and a higher pH (7.7 to 7.8) to lower the amount of carbon dioxide in the water.
You should also maintain the CYA (Cyanuric Acid) at 60 to 80 ppm, and the CH (Calcium Hardness) high enough to reduce the risk of dissolving calcium carbonate out of the plaster.
Another possibility is a leak and the auto fill introducing a lot of fresh water. You should verify if the auto fillers are adding a lot of water.
Adding Borates can help, but I would wait to see what you can resolve without them first.
Can you post a full set of readings, and the TA of your fill water?