Hi @mate2go,
leave the 340 package installed and install the following package (or at least try it), the terminal command (Ctrl + Alt + t) is as follows:
sudo apt install nvidia-driver-390
This is on the assumption it will work for you???. 
Hi @mate2go,
leave the 340 package installed and install the following package (or at least try it), the terminal command (Ctrl + Alt + t) is as follows:
sudo apt install nvidia-driver-390
This is on the assumption it will work for you???.