Fluctuation in Google rankings is actually fairly common, especially when you’re in the bottom half of the first page or on pages 2,3,4,5, etc. The rankings also fluctuate right after you first enter into the top 5 results. It’s only after you’re consistently in the top 5 results for 2-3 months, that the rankings tend to stabilise. So some amount of fluctuation is a natural part of the Google algorithm, since it customises search results based on the user’s location and history. The only time you need to really worry is if all your web pages lose rankings fairly drastically.