Flickering in KDE Plasma 5.4

I was having the same issue but with a different graphics card and a different OS (Fedora 22 KDE spin). I just came across this:

https://ask.fedoraproject.org/en/question/69542/fedora-22-full-screen-tearingflickering-under-kde/

which has several options that seem to solve the problem under Fedora at least. I'm trying option 3 and it seems to be working so far, and the first two options are things that can be changed purely through the kde system settings app (shouldn't be specific to OS).

Hope this helps you!

EDIT: In an effort to make this answer better, I'm putting a summary of the link given above here. To solve there issue there are three options

1. Go into System Setting > Hardware > Display and Monitor > Compositor and change the tearing prevention to "Full screen repaints"

2. As an alternative, in the same menu change the compositor to XRender

3. If you'd prefer to not do either of the above, create a script file in /etc/profile.d named kwin.sh (or anything ending .sh really). In that file place the line export KWIN_TRIPLE_BUFFER=1

This is recommended if your .xsession-errors file mentions that Triple Buffering is not available. To find this, I just opened the file in kwrite and searched for triple, sure enough it said not available. After making the file, that message is no longer present on subsequent reboots.

I have flickering too. I also found that changing compositor settings fixed it, but only temporarily. The reason this worked is that it restarts the compositor. I don't have a full solution yet, but restarting the compositor with SHIFT+Alt+F12 twice is an efficient workaround.

On my system Kubuntu 18.04 with NVIDIA proprietary driver none of the solution listed int he answer worked, for me disabling "Sync to VBlank" make the issue stop for now (a reboot was needed)