Comment # 14 on bug 1201398 from Stefan Dirsch

Hmm. Shot in the dark. Try with

Section "Device"
    Identifier "modesetting"
    Driver "modesetting"
   Option "AccelMethod" "none"
EndSection

instead of

Section "Device"
    Identifier "modesetting"
    Driver "modesetting"
EndSection

(i.e. add this Option line) in /etc/X11/xorg.conf.d/90-nvidia.conf file.