[I have a blog that it’s on “dancing” with Google. One time it’s on 4ºposition on the 1st Google page. Another time it’s on page 460. Don’t know what else to do, lots of backlinks on-page SEO, done all. Even added another year, it lasts 1/2 days on page 1 then Google sends it to 460+ again for 3/4 weeks.]
This is the nature of a blog. The more often you update it the more often you will see the rankings fluctuate. Rankings change because of the content changes.
What is happening is that when you add new content that content gets indexed and Google will apply QDF to the fresh content. When this temporary boost wears off your page then sinks to it’s the current earned position.
QDF is the term Google uses for its freshness factor. It is an initialism for the term: Query Deserves Freshness. It is a well-known factor in SERP ranking that was first officially revealed by Google Scientist Amit Singhal in a New York Times article back in June of 2007. Google engineer Matt Cutts also confirmed the existence of QDF on the official Google Webmaster Help Youtube channel:
The range of fluctuation you will see, due to QDF, varies based on the amount and strength of competition. The greater the competition the larger the swing in rankings will be.
Again, nothing weird here, just the type of ranking fluctuation one should expect to see for a more competitive keyword.