Check the total alkalinity. Low alkalinity will cause the pH to change easily. Which will make it hard to stablize especially if you have a salt system. Total alkalinity indicates the degree of the water’s resistance to change in pH. Another way to describe total alkalinity is to say that it is the water’s ability to neutralize acid. The ideal total alkalinity is 80 - 120 ppm for plaster pools. I usually aim for 90 ppm for my pools. Use sodium bicarb or soda ash to increase the alkalinity, soda ash will increase the pH a lot more. As others have suggested you may be getting interference from high free chlorine levels. If your free chlorine is above 5 ppm you will need to dilute half of your test water with distilled water to get an accurate reading of the pH. After that use the acid demand test to determine the correct amount of acid to add.