Option 1: Install GNOME System Monitor with the Software Center
Open up the Software Center from the Applications menu
Search for GNOME System Monitor
Select GNOME System Monitor from the results and click install.
You can now launch the GNOME System Monitor. It will look something like this:
Option 2: Install the GNOME System Monitor in the Terminal
One terminal command you can use is called 'top'. The link is here. I personally would use that and leave the terminal window open and just monitor it to see what process is causing the issue. This will at least give you an idea to where to start looking.
Maybe this link will help you.
The terminal is capable of showing you running processes via the top command.
All you need to do is open a terminal, whether on your desktop or, if things have frozen by pressing CTRL+ALT+F1 - F12, logging in and entering the top command.