Major keywords appearing in SERP for 1-2 weeks on a regular basis and then vanishing

Hi,
I am the webmaster of SexyFun.co.uk. I have been experiencing fluctuations in rankings on some major keywords over the past 3-4 months. Our home page starts ranking on major keywords that we are targeting but those rankings stay only for a period of around 1-2 weeks and then the rankings disappear. This has not been a one-off incidence as it has happened for the 4th time in less than 4 months. Our site does not have a Google penalty, nor have we done excessive promotion or major overhauling onsite changes to the site during this period or even before that. I would request people to please suggest as to what exactly could be the problem and what could be a probable solution.

 

I also have this problem

My keyword was #5 for around 2 weeks and now it is off top 100.

The weird thing is that it is not a very popular keyword (around 1000 searches/month)

The websites that are in the top 10 have ranking of over one million.

My ranking is 292 000. no penalty. lots of daily new unique content...

Just don't get it

 

I'd go for the long haul, I've read many times sites will fluctuate within the first six months before they settle. My rule of thumb is six months to gage where I am.

 

Google likes to bump "fresh" for a while then let it settle in.

 

Any concrete proof on this?