Cause for fluctuating rankings
 

There are some questions that I would like to ask. For instance, there is a well-optimized site with a very strong visibility on Google. Like if it is on the 2nd rank of Google. Then it becomes the number one position in Google for a month. Then it goes down to number two again. What do you think is the cause for such movement? Do you think it is normal or a weird one from Google? Is it possible to pay for SEO and then control the ranking or can I get it short term rankings on top from a good PR site renting a link?

There are many reasons that a website's ranking fluctuate like not getting the authority backlinks or not updating the content etc, etc....

It seems that your competitors are working harder than you.

You need to spy on your competitor strategies before strategizing your SEO campaign.
You may take help of some keyword tool for this. I prefer Keyword country to rip my competitor keywords. It even alerts me when my competitor changes strategy.

Keep doing SEO rigorously and you you will soon regain your rankings.

The fluctuations are because of the fact that just like you other webmasters are also trying to achieve higher rankings. Thus if they use better techniques then you they can get better positions on search engines.

Well in your case it has happened because your competitor also trying to get number one position and regularly doing link building and new activities for getting a high rank.