Nginx + PHP-FPM = "Random" 502 Bad Gateway
I think your max_children is too low
<value name="max_children">3</value>
Also you can try switching from TCP port to unix socket for php-fpm:
php-fpm.conf
<value name="listen_address">/tmp/php.sock</value>
nginx.conf
fastcgi_pass unix:/tmp/php.sock;
Elaborated:
- Unix sockets are ~20% faster
- You don't use time-wait sockets for each connect