I have no idea why, but if I'm browsing internet with Chrome or Firefox, some minutes later, the browser start behaving wierdly, like almost freezed but not totally. Then I try to open a new tab, but it doesn't work (but it repaints the button, like it's receiving the click event). Then I try to close the browser, and I can't. Then I try to reboot the system going to the reboot menu of Gnome. I see some of the icons have dissapeared, and if I click on the "turn off" button, it doesn't work and the system freezes completely. Sometimes I even don't have time to reach the "turn off" button.
It takes longer to happen with Firefox, and it's more frequent with Chrome. What's going on here?
If I don't use the browser, the system works without issues.