This answer is from my experience... The answer as many have already told you, the right thing is to keep build backlinks, even directly to your money site. If you stop after this drop it will hurt more.
A have an offline business website that I rank on second page for the main keyword (11.000.000 search/month ) and on the first page for around 1000 secondary keyword only with a few bookmarks, many directory submitting and a lots and lots of SB blog comments backlinks directly to the main website. After a few month of this strategy I decide to go and test a more white hat strategy and slower down things (when I say a lot I mean 5000 backlinks per week, the posted ones, since I blast list of 10k a day). So I move the blast to secondary website and things like that. I do that for a 1-2 month... The result... disaster.. I drop on all the keyword and the phone stop ringing.
Now that I get back on the old strategy.... big amount of backlinks build with SB to the main website, it start to rise again..... I have a very aggressive strategy, my website get some big dances like yours but if I keep what I was doing it come back, some times even higher.
After this I read somewhere here that the best thing to do when you drop down is to keep building backlinks (the ideea was something like G will see those backlinks keep building so it will consider that the website is good not because is on first page, and no matter where it is on serp it get people to talk about---> this was the idea not the exact explanation).
So.. in your place I will keep blast the hell of it.