NVidia + KDE VSync problem

[Hexorg]

I can't get VSync to work for more then about 2 seconds with NVidia and KDE (using their binary driver).

VSync is enabled for opengl and XVideo in nvidia-config.

If I go to KDE's effects settings and switch between the Qt graphics System (Raster or Native) I get VSync to work for about 2 seconds (FPS effect shows 59fps) and after that VSync stops working and FPS monitor jumps to around 90FPS. It doesn't matter if I select Raster or Native - the VSync starts working when I change that value, and stops after about 2 seconds. I tried to play with some other options in the Advanced tab of System Settings -> Desktop Effects, but I can't change this behaviour.

Has anyone heard of this odd behaviour? Are there remedies?