How do I diagnose a unstable installation on ubuntu 16 server?
Unless you are using Drupal 6, I suggest you start again from scratch and use/install PHP 7.

Drupal 8 and 7 (since 7.50) support PHP 7, which is twice as fast as PHP 5.

sudo apt-get update
sudo apt-get install apache2
sudo apt-get install curl
sudo apt-get install mysql-server
sudo mysql_secure_installation


Would you like to setup VALIDATE PASSWORD plugin? n

Change the password for root ? n 

Now, for the rest of the questions put y

sudo apt-get install php php-cli php-curl php-gd php-mbstring php-mcript
a2enmod rewrite
sudo service apache2 restart


Now, Install and Secure phpMyAdmin and create a database with and a user with privileges. Do a fresh Drupal install and make sure you get no errors in the install screens.

Once all ok, dump all your DB tables and import your site. Make sure your Drupal directory contains all your modules and the files folder contains all your images and files of your importing site.