High rankings and impression inside first month, I'm going to take a wild guess and say blackhat links...
Penalty and dropped from main indexed KW's, again I'm going to take a wild guess and say blackhat links...
I'm guessing you hit your site with 1000's of links right after posting the content with a bulk of their anchors the exact match keywords that your domain targets.
That roller coaster is usually what happens when using blackhat strategies directly to your money url, hint hint. For a couple weeks your serps are great then BAM! flatline.
Clean up your links pointing towards your site. And work on getting some articles for guest posts done from authority pr1+ blogs in your niche. Once your site is back and gaining authority in your niche then you can send SOME higher quality spam links as is natural for some authority sites to have some spammy sites linking to it. I bet wikipedia or about has many spammy sites linking to it yet those links help it as it's known as an authority site...