I am having a similar issue, and I think the high TA fill water is also the culprit. My fill water is TA 140, which I just measured for the first time today. I can knock my PH down to 7.0 or less with a gallon of 14.5% MA, aerate over the weekend with my aerator water falls, and bring the TA down to 70 and PH to 7.6, but then in a few days or maybe a week I am back above 7.8 and TA 100. It was making me crazy until I checked the fill water today.
Any suggestion what to do about TA if your fill water is high TA? I think the answer will be "stock up on MA". Ugh. I was hoping to add borates, but now I am thinking that will just make it harder to bring down the PH and TA when they inevitably drift up.