This is a "legit" site and most links a genuine whitehat backlinks - Some have quite high authority. I have been spicing them up a bit with a pbn I have been building, although I'm not even sure it matters much since we have good results through whitehat techniques. We may have been building links relatively fast, so as to trigger something with Google. Not sure about that.Google dances a lot when it sees new backlinks and all. But how is your backlink profile and what sorts of links are you building?
I had the google dance going on for 9 months! Everything between page 2 and 9. Was really annoying. But now I won the battle and rank on 2. Stay patient...
Pretty standard for newer websites, and even at times older ones can see the same type of fluctuation. I have sites that are years old that have a similar ranking graph.
Just be sure everythings in order though as Panda keeps getting rolled out and Penguin is right around the corner.