I believe you should tweak PHP_FCGI_MAX_REQUESTS. You can try 5000 for instance. In order to get this working you need to kill all (stop nginx, pcp_cgi and killall nginx, the same with php_cgi or whatever you called the init.d script) the processes running and start php_cgi and nginx again. I'm saying this because I just read your other thread where it does nothing PHP_FCGI_MAX_REQUESTS on your server.

I also run a wp blog and vb forum on my site. I'm getting over 170k uniques per day. I run mysqld on a separate server. I have 8 workers. 15 children. I'm currently tweaking PHP_FCGI_MAX_REQUESTS.1000 seems to be too low for me since php-cgi processes get restarted too often.