Dear Sir or Madam,
Your problem is probably related to RAM memory, at least that is my first bet.
Tools and technologies I am knowledgeable of or familiar with include: *NIX System Administration, Virtualization Software (Oracle VirtualBox), nginx, Apache, MySQL, FTP, WordPress, Ghost, Git, pfSense, FreeNAS and more, so I am positive I can fix your problem.
I can check if anything on your system is taking more RAM than it should, maybe add swap file to your system to increase available memory.
I am looking forward to hearing from you.
Regards,
Predrag Mijatovic