Ubuntu server won't stop running processes

I have an Ubuntu 16.04 server with ISPConfig 3.1 and many websites installed.

For some reason the server started working too much on some processes with high load, and it gets too slow from time to time. I don't know what the running processes associated with this problem are, but it doesn't work well anymore.

I want to identify the running processes in order to figure out the solution. Hopefully someone can shed some light.

Thanks in advance.

 

Usefull commands are (you may need to install them): free -h, htop, dstat.
Useful Internet Search Engine string:

Code:

linux find which process causes high load

 

The problem is that it won't let me use them because it's stuck and not responding

 

Boot the server in single user mode, then first check if disk is full:

Code:

df -hT

Then boot normally, and before the host gets stuck, monitor with the commands showed what is happening. For example htop in one terminal windows, dstat with suitable parameters on another.