At current position droping of rank is due to new update by google named as penguine. So it might be one reason if everything else will be all right. And if penguine is one reason than try to fix problem as this update is all about over optimization and using black hat techniques for optimization.
If this is also not one reason than its generally happens in google that fluctation is normally taken place. So don't worry about it just concerntrate on your work and keep continue promotion and link building with quality sites.