Firstly, does nvidia-smi work?
Secondly, look for an X configuration file under /etc/X11/xorg.d/ (or as /etc/X11/xorg.conf) and see what its contents is. If you have a file for the nvidia device then you can add Option "Coolbits" "8" into its [Device] section.
If not, you can create a minimal configuration file that also contains the coolbits option.