I remember reading on the old forum that each pool's ph tends to be different. Some tend to drift up and others down. Why? One thing is the type of chlorine you use.

Bleach tends to cause the PH to run up. Some pools faster than others. Keep up with the Muratic Acid. I don't understand the dynamics of PH and scaling on plaster, but you may need to lower your PH down to 7.2 and let it drift up to 7.6 and then add more acid to prevent the scaling since you are getting that at 7.8

Good Luck!