i have a blog site which is http://www.bonecancerprognosis.org/ i had started work on this site three months back i gained instant ranking at number 3 in Google but now the condition is that the result of my website is not seen in Google for couple of days and then appears again and within couple days it disappears and then appears again
I have also noticed this happening with a couple of old sites i have... The only conclusion iv'e come to so far is if your site does not get updated very often with relevant, fresh content then i think google will sometimes ignore your old site and concerntrate on the newly updated competitor sites who are targeting the same keywords! Just my conclusion - take it or leave it
Sounds pretty typical for a 3 month old website to me. Some like to call it being in the sandbox. Most new sites do not have the trust rank to move directly to the top of the rankings and stay there. Keep building links and content and eventually you'll get the desired effect of moving up.