If your SERP (Search Engine Result Pages) rank fluctuates, don't be alarmed. Everyone's rank fluctuates, and that is the nature of search engines. You may be happy to see your site rank highly on most coveted keywords, but don't expected to last forever without constantly updating your site. Search engines are constantly tweaking their algorithms and new sites are constantly being added, so SERP ranking fluctuations are expected. Also, your competitions are updating their sites to achieve better rankings.

About once in every 30 days, Google recalculates the page rank of each web pages, and re-indexes the SERP for all keywords. The Google Dance is the time period between the start and end of the re-indexing. During Google Dance, the search results are different on www.google.com, www2.google.com, and www3.google.com. The www2 and www3 are known to be Google's test engines. The dance normally takes about 4 days.

Even when no Google Dance is in effect, SERPs may be different for same keywords at different time of the day. This may be due to hitting different Google servers between searches. Search results may be different on different Google servers, and geographic location and load balancing potentially makes users land on different servers.

You as a website owner should concentrate more on targetted traffic and conversions rather than pure SERP rankings. As you enrich your website by adding more relevant contents and building backlink to your website, you are ultimately attracting more targeted and qualified traffic.