Default_server catch all block not working

[lockev3.0]

Hi, getting quite desperate at the moment about this.

I have a server serving several subdomains; actually proxying'em to
apache2. So I have main nginx vhost file with all server_name's added.
Such server names match the defined ServerName's and ServerAlias'es of
Apache.
Beside, I have one more file to  define a default_server catch all block
like follows

server {
        listen       80 default_server;
        server_name  _;
        error_log       /var/log/nginx/000default-error.log error;
        root /var/www/forvo.com/_templates;
        error_page 404 /404.html;
        location / {
                #return       444;
                return 404;
        }
        location = /404.html {
                internal;
        }
        #rewrite .* http://myfaultysubdomain.com permanent;
}


As soon as I enable this file, everything works as expected, however, in
less than one minute (my site is always being visited), all requests
(good subdomains and bad subdomains as well) start to get 503 response.
No log at all seems to change when this happens.

Any clue to what may be happening pleeeeeease ......it's been hours,
during days, trying to figure out what the h@$!! is going on

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,221572,221572#msg-221572