How to find what cause 503 error?

[baalchina]

Hi,everyone.

I am using an nginx 0.6.32 and an apache 2.2 server, running a web
forum, using php and mysql.Apache listen in 127.0.0.1:81,processing
php requests.

It works fine for nearly two month. But today I found my server have
503 Service Temporarily Unavailable too often.After refresh my
browser, the web page quickly became normal.

I checked my server(centos 4.4), free disk spaces,cpu load,all of them is fine.
I also restart my nginx and apache, still doesn't work.

And I checked my nginx & apache error logs, still nothing valueable.


So, how can I find what cause my server have 503 error? A debug log can help?

Thank you.

-- 
from:baalchina