We have had the #1 postion on Yahoo for most of our keywords since Yahoo switched from Google. Sometime over the holiday weekend we went from #1 to no where for our index page. We still have interior pages that rank fairly well (10-15 position)
If I search for www.oursite.com I still find the index page it just no longer ranks for any of the keywords.
We did switch servers about 3 weeks ago so maybe that is a factor or maybe there is some pentalty at play here. We do not have any affiliate sites, are not crossed linked, no hidden text, etc, etc. Do not believe we are overoptimzed either as google ranks us in the top 3.