Random freezing on any app

Since 3 weeks ago I'm experimenting an annoying random freezing of my computer that forces me to reboot the system. I've deleted the hardisk up 3 times, and created a new account reinstalling each software step by step (without Time Machine).

It seems that is not a hardware problem. It tends to happen when I surf, or when I've the browser open. No matter the browser (Safari, Chrome or Firefox). I don't know what to do.