A few things that will help iwth the pH....
First lower your TA to around 70-90 ppm. Once this is accomplished bump your CH up to about 300 ppm. (The reason this needs to be done is complicated so I am not going to give the full explanation here but it's basically because you have a plaster pool).
Second, once your TA is in line keep your pH at 7.6 and not lower. The lower you put the pH the faster it will rise (we use this fact to our advantage when we lower TA). When you pH rises to 7.8 use the acid demand test in the K-2006 to bring it back down to 7.6 and not lower.
Finally, adding borates to 50 ppm to your pool can help with pH stability tremendously. If you decide to add borates there is a very good tutoarial on how to do so here.