pH has fluctuatoins because of the acids in the water, usually bromine, chlorine, shock.
While the bromine is in the water it is constantly tearing down the pH and making it acid.
If you adjust the "Total Alkalinity" to a level where it stops pH fall, it will solve a lot of problems for you.
Consider the Total Alkalinity as the crutch that keeps the pH from falling. However, if the TA is too high it will pull the pH up too high.
The idea is to have constant use in the spa and constant amounts of bromine, but this is never the case. You need to test the water about twice a week for a spa that has heavy use.
The TA is normally set to 100 PPM and the pH to 7.4 with bromine.
You may consider a much easier method of spa care called Eco One or Instant Ions. http://www.spaspecialist.com/