Thanks for all your replies. WHT is really a good place to discuss. : )
The new server is our own. Meanwhile, we keep our forum on a shared hosting for server load issue we met before. That's the reason we buy a our own whole server, then we don't need to host our sites seperately. We planed to move all our stuff (the other 3 domains, we only host one big site and 3 very litte site) over the new server within two weeks.
However, we stopped to move the rest sites now because our main big site is facing the rank issue.
We don't want to blame our new mangaged webhosting, I believe that they can't do anything about this.
However, moving to a new own server is the only coincidence we can figure out to our Yahoo rank descending.
You all may be right, I should check what the other top rank sites are doing now.
But it really hurt for us to lose so many ad views within a week. : (
As for google, we wonder if they have ever changed their ranking?
Many related site down for couple days a week, google still gave them good ranking. Even better than us. Even we tried to write to them and showed that we have much better reputation or reference on Yahoo knowledge and much higher traffic, Google still do thing to improve our ranking.
It may be a goog news for us because the percentage of our members from Google is much much less than from Yahoo. That's why we care Yahoo more than Google and google never tried to fix their ranking anyway. Our members are from Aisa mainly. In fact, we are considering to block google bots on our robots.txt or .htaccess for that they are giving us bad ranking but sending a lot of bots everyday.
Thanks for all the suggestions here, our Yahoo ranking are still not normal. Perhaps, we need to rewrite all our pages or just change another domain or pay for overture.com.
We once thought that if overture wants us to pay for the keywords marketing fee again because we once spend some money to use overture marketing.
We have not pay for the marketing fee for nearly 6 months.
And we are sure that whether we pay for Google keyword or not, nothing change to our ranking. For we once paid some money to google as well. We think Google is not worthy for us to pay more marketing fee.
Finally, we decide to pay overture keywords again. After sending this posting, we will do it. Hope that works. After all, we can't afford so much ad view loss.
It is really the most painful month for our budget and cash flow so far.