I have had the same experience as you on TA. Mine stays in the 70-80 range. Which is fine with me and my pool because the Ph stays at 7.5 almost religously. I like you have added soda to raise TA and it will come up to about 90 then settles back down to 70 in around a week. Basically all I add to my pool is bleach and some polyquat once a month. This is a IG 20x40 vinyl pool with no swg.
I contribute mine to having my returns pointed upwards to ripple the water. May or May not have anything to do with it. The pool tends to stay at a TA of 70 and a PH of 7.5. I have even changed the ph just to see what happens and it will settle back at these numbers given several days. That being said it is coming a gully-washer outside right now and I'm sure I will have to get everything straightened up this evening. It happens everytime I post something about a constant PH. I guess I should just keep my mouth shut!!!