yahoo 301 dangerous?

Today I found my site disappeared from the first 3 pages on yahoo on many keywords. My website used to rank very high from Dec. 2004, and I have rencently been useing CO-OP (about 30k weight using about 8 keywords) to advertise my website. 2 weeks ago, I found yahoo does not handle url very well, for example, it show [my web site].com/url.html instead of www.[my web site].com/url.html, so I added 301 redirect in php, and if it founds the url is not start with "www" then it will redirect it to full url.

Rencently I found some of my keywords ranking became worse, and 3 days ago, yahoo showed my website as [my web site].com/url.html on my main keywords, it used to show www.[my web site].com/url.html . Today I found lost these keywords' ranking.

Could it be yahoo algo change or my 301 redirect is wrong ? or co-op did a negative effect on it

 

What I think I've found with Yahoo! is the right way to handle 301s is to forward the link for some amount of time, then 404 the old link.

Everything looked like we were set up at the new URL. But looking at Yahoo!'s cache, I would see that it actually had the old URL.

We only started 404ing Friday afternoon but are already seeing some positive results from it.

 

Have you checked the Server Response Header for the Non-WWW URL? What is the URL?

 

This is the code I use

$host=strtolower($_SERVER['HTTP_HOST']);
if ($host=="[mysite.com]"){
$url="http://[www.mysite.com]".$_SERVER['REQUEST_URI'];
header("HTTP/1.1 301 Moved Permanently");
header("Location: ".$url);
header("Connection: close");
exit;
}


I saw some people reported yahoo update here
http://forums.seochat.com/t28390/s.html

maybe it's algo change

 

seems fine to me