The quesion will be kill me

[refercon]

Using nginx make proxy server,  the backend server are nginx,apache
Sometimes,the web is not ok !

An error occurred.
Sorry, the page you are looking for is currently unavailable.
Please try again later.

If you are the system administrator of this resource then you should check
the error log for details.

Faithfully yours, nginx.

Moretimes the web is ok，little times is error...
The nginx error log is:
2013/01/14 16:02:26 [error] 31316#0: *1154349 recv() failed (104: Connection
reset by peer) while reading response header from upstream, client:
61.186.137.140, server: m.xx.cn, request: "GET
/pnp4nagios/index.php/graph?host=sxcq-web&srv=PING HTTP/1.1", upstream:
"http://10.2.4.10:80/pnp4nagios/index.php/graph?host=sxcq-web&srv=PING",
host: "m.xx.cn", referrer:
"http://m.xx.cn/pnp4nagios/index.php/graph?host=sxcq-web&srv=PING"

2013/01/14 16:02:26 [error] 31310#0: *1161676 recv() failed (104: Connection
reset by peer) while reading response header from upstream, client:
123.151.148.201, server: www.xx.gov.cn, request: "GET
/plus/view.php?aid=3402 HTTP/1.1", upstream:
"http://10.2.4.4:80/plus/view.php?aid=3402", host: "www.xx.gov.cn"


I checked backend server's log ,,, No erros in it ...

My nginx proxy conf is:

Http's section:
http {
    include       mime.types;
    default_type  application/octet-stream;
    sendfile        on;
    tcp_nopush     on;
    tcp_nodelay on;
    keepalive_timeout  120;
    client_body_timeout 300;
    server_tokens off;
    send_timeout 3m;
    gzip  on;
    gzip_http_version 1.0;
    gzip_min_length  1000;
    gzip_buffers     4 8k;
    gzip_comp_level 6;
    gzip_types   text/plain image/gif image/jpeg image/png  text/css
application/json application/x-javascript text/xml application/xml
application/xml+rss text/javascript;
    client_max_body_size 64M;    
   server {
        listen       80 default;
        server_name  _;
        return 403;
    }
Proxy's section:
upstream xx{
	server 10.2.4.11;
}
server {
        listen 80;
        server_name www.xx.cn;
        #charset koi8-r;
        #access_log  logs/host.access.log  main;
        location / {
            	root   html;
            	index  index.html index.htm;
                proxy_redirect off ;
                proxy_set_header   Host $host;
		proxy_set_header X-Real-IP $remote_addr;
                proxy_set_header X-Forwarded-For
$proxy_add_x_forwarded_for;
                proxy_set_header Accept-Encoding "";
		client_max_body_size 1000m;
		proxy_connect_timeout   15;
                proxy_send_timeout      600;
                proxy_read_timeout      600;
                proxy_buffer_size       512k;
                proxy_buffers           8 512k;
                proxy_busy_buffers_size 512k;
                proxy_temp_file_write_size 512k;
		proxy_next_upstream error timeout invalid_header http_500 http_503
http_404;
		proxy_pass http://xx;
		include attack.conf;
        }
	location ~ .*\.(html|js|css|jpg|png|gif|flv|ico|swf)$ { 
		expires max; 
		root /home/nginx_cache/www.sxcq.cn; 
		proxy_set_header   Host $host;
                proxy_set_header X-Real-IP $remote_addr;
                proxy_set_header X-Forwarded-For
$proxy_add_x_forwarded_for;
                proxy_set_header Accept-Encoding "";
		proxy_connect_timeout   15; 
		proxy_send_timeout      600; 
		proxy_read_timeout      600; 
		proxy_buffer_size       512k; 
		proxy_buffers           8 512k; 
		proxy_busy_buffers_size 512k; 
		proxy_temp_file_write_size 512k; 
		proxy_store_access user:rw group:rw all:rw;
		proxy_temp_path /home/nginx_cache/www.xx.cn;
		proxy_next_upstream error timeout invalid_header http_500 http_503
http_404;
		proxy_store on; 
		if ( !-e $request_filename) { 
			proxy_pass  http://xx;
                } 

         } 
     
        error_page   500 502 503 504  /50x.html;
        location = /50x.html {
            root   html;
        }

    }  



Pls help me... Thanks!

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,235031,235031#msg-235031