If youre targeting for highly competitive keywords. it might be a sandbox.. if not... thats only a google dance
i got crushed by google in the same way as well, no malicious code though .. and it's not for a limited set of keywords, its for a zillion different random searches as that's how the traffic comes in. As if it were getting hit by some filter..
I've since upgraded to WP 2.05 and changed my URL format to a more SEO friendly version. We'll see how long it takes for me to come back up.