Hi Jimany,
The overall impression seems, Ubuntu 18.04 is consuming a large percentage of your computer’s available resources, i.e video graphic and memory. The graphic card will drain resources to function.
To eliminate this possibility, you need to experiment by installing a light weight desktop. First of all save all your data to an external drive. Before installing.
Go to this page: Download Linux Mint 21.1 - Linux Mint and download LM19.1 Xfce 64bit, leaving the 32bit as a fall back situation, if the 64bit proves too heavy.
You should be able to install using your usual method, once post install updates and upgrades have been done, reboot, then go to “The Driver Manager” in System Settings and select the Nvidia 390 driver, this should resolve if all issue, most of them. Then reboot.
See this post for Driver Manager & Nvidia
System tweaks
Firefox tweaks
The Xfce desktop is a fine GUI, that doesn’t the same resource requirements as Ubuntu, this particular distro (Linuxmint) uses Ubuntu’s platform to deliver its choice of desktops. It is very well packaged and is ideal for modern through to veteran computers.
I have included different links to aide your progress.
The result should very promising, and you upgrade your hardware with extra ram and a SSD drive, you will very pleased with the increased speed and ease of use.
Edit 21/02/19 tidy up links and typos