Hello. I'm new user KDE workspace, so I've a problem. Why KDE 5 causes of my screen flashes? This problem I've on Kubuntu 15.04 and Debian Stretch.

Hey, I was having similar issues on my system. It seemed to be an issue with the Open GL 2.0 Rendering backend for the compositor. Ether do a search in you launcher for compositor or in Your KDE system settings go to display and Monitor, click compositor. Change it from openGL 2.0 to 3.1 and I set it to EGL. Once I did this the flashing seemed to stop. My computer is running on an AMD integrated chip. Not sure if it will fix your system but it sounds exactly like my issue.