Ubuntu complete freeze

Hi, I'm writing this post because I've a problem which appeared on Ubuntu 14.04.4 LTS and still happen on Ubuntu 16.04 LTS, both 32 bit and 64 bit. Indeed, after some time (without any criterion) my PC freezes completely, and this makes impossible to use the mouse and to open the shell, so I always have to shut down it with the power button. This thing happens independently on what I'm doing with my PC, but it happens more frequently when I'm watching Youtube videos. When I switch on my PC, it doesn't report any error, neither in the log.
If this can be helpful, my PC is an Acer ES1-512-P2C7.
I hope you will help me.