I've tried to find any common program or activity that could cause a crash. All of a sudden, I receive a black screen. I press ALT F2, login and reboot. This happens within Kubuntu or Xubuntu after about 1 hour of use.

Linux 3.19.0-25-generic
KDE Plasma Version 5.2.2
Intel(R) Core(TM) i7-4600M CPU @ 2.90GHz
Lenovo Laptop T540P

If I may provide additional information, please inform.

Thank you.