google did a major update and even good authority sites dropped all the way see the official google blog
I also have the same issue. My site working just fine at second last month but in this month get rank down I don't know what is the reason?
It may due to Google Webspam update done on 24th April. You are not alone. Large number of sites got affected.
It is due to the new Google update, which penalized sites for over optimization and too much of promotional activities done via forum postings, blog commenting to improve their keyword ranking.
Google change It`s Algorithem it`s change panda algo. now include Pentium algo for SEO so check your SEO.
i do my seo work thats not the problem the updates come fast and i use all different back linking ways