By default Apache only allows upto 150 connections, if the number is over, you will be in the queue. It will look like the server is down.

The limit for server is usually configured in the following files:

/etc/apache2/apache2.conf
/etc/apache2/mods-available/mpm_worker.conf
/etc/apache2/mods-available/mpm_prefork.conf

This is the content to update: