Nginx+apache2 ubuntu server crashed with several hundreds of visitors

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http://serverfault.com – I have Linode 768mb RAM server on Linode. And i have Wordpress blog. On my server installed ubuntu, nginx as frontend and apache2 as backend. And i have APC and memcache modules. Sometimes site is crashing. But CPU usage of server is less than the critical levels (only max 60-70). However, during site crashing i can see the critical levels of hard disk I/O usage. I have read that this may be related to incorrect settings of mysql. My nginx.conf: worker_processes 2; events { worker_connections 1024; # multi_accept on; } http { sendfile on; #tcp_nopush on; #keep (HowTos)