On my 12,000 gallon pool I started with an alkalinity of 370 and it's down to 130 after 4 days of constant aerating with a 12 ft long 2 ft tall cascading waterfall, two deck jets and a foam jet in the beach entry. My pool also has eaten 5 gallons of muriatic acid in the process.

I think your problem is the aeration, or lack thereof. You need to really move that water around and out of the water to get some good air in there. You can lower your pH even more (a cup should bring it down about 0.2 on the pH) and jsut keep an eye on it.