Use proper pH down, Nitric or Phosphoric acid. Acetic and other organic acids aren't necessarily going to hold your pH stable in and aqueous solution in the presence of other compounds.

When you shake it you are allowing whatever is in the atmosphere to react with the contents of the liquid. Trust the first reading. I hope you are cleaning out the test tube (rinse 3x and then pre-rinse with the liquid to be tested) between readings.