Where's your Calcium Hardness level? Are your sure it's in the 200-400 range? Do you have an SWCG as well? Or are you chlorinating with cal-hypo, calcium hypochlorite?
Also, what's your CYA (Stabilizer) level.

Try NOT raising your TA and letting it just go down, say to 60 or 70. See if your pH stabilizes.

IF you CYA is low, you can try using Tri-Chlor tabs to chlorinate. They are VERY acidic and add 6ppm of CYA for every 10ppm of FC.