From what you have posted, it seems you have triggered an "Algorithmic Penalty". Most likely the "Penguin" as it will demote pages not just whole sites. That algorithm is that granular in how it treats pages.
Stop your link building, then inspect your profile and remove the bad links.
Just an FYI, you may have done irreputable damage ( burned the domain ), if so you will need to get a new domain and start over.
Unfettered link building has destroyed many sites.