You should have some type of analytics software setup so that you can see exactly where the drop in Yahoo traffic is coming from. It could be that all of your Yahoo rankings are lower or it could be that 2 or 3 important keywords dropped in ranking.

According to Yahoo you only have about 156 backlinks. This simply isn't enough to get decent rankings. You need to work at building more backlinks with good link anchor text.

I'm not sure if it will help, but you might want to specify your sitemap file in your robots.txt. Also try submitting your sitemap to Yahoo directly.

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