I have kubuntu 15.10 on my desktop PC with a radeon r390. It happens sometimes that the desktop freezes completely and there is nothing that I can do to fix it but hard reboot. It can happen anytime and I can't find a pattern. I installed fglrx and it works very fine (beside this issue) and I had the same problem with the open source drivers.
I really don't know what to do anymore. I have looked around to find a solution but I can't find anything.