Server crashing every hour or so returning a 500 error
My server "4 GB Memory / 80 GB Disk / TOR1 - Ubuntu 16.04.2 x64" was working fine until I've installed the page speed module for Apache: https://www.digitalocean.com/community/tutorials/how-to-get-started-with-mod_pagespeed-with-apache-on-an-ubuntu-and-debian-cloud-server. I didn't install the Nginx module since my web server is apache.
I didn't change anything else, kept all the default settings, but since then, all the websites on the server crash every hour or so ... then when I restart Apache everything works for another hour. Am I missing something here? By crashing I mean, when I go to the website, it returns a 500 error.
I've installed Vesta CP on the server. What I find weird is that I can still access the cpanel when the other websites are down, which indicates that the server is not down itself but something that has to do with the virtual hosts.
Maybe one of you have had that problem before.
Thanks in advance.