I get that problem with 60hz over HDMI. 1440p.. Apparently it's quite common. Mine just makes the monitor black screen. Displayport is fine, but yeah, something odd going on with Nvidia drivers and 1440p 60hz ! Apparently there is no fix from Nvidia either, so I had to do the following.
Enable DSR. Enable all of the factors. Create a profile @ 59hz, apply. Now if and when you select the 60hz profile (which it defaults to every chance it gets) the oddness should not occur.
The biggest gripe I had was that every time I changed it to my custom resolution it kept switching itself back. So for example run Valley, exit, monitor goes back to default 60hz. Run something else (game, bench etc) black screen have to reboot. Adding DSR and enabling the factors stopped it doing that.