Why not see if your pH will stabilize at 7.6 or 7.8 with a TA of 70 or 80? It could be that your TA is falling because of your constant additions of muriatic acid to keep your pH at 7.4.
My TA was around 110 until I held my pH at 7.2 with muriatic acid for about a week. At the end of the week...