U2520D at 2560x1440 as a second monitor turns itself on and off

I am running Ubuntu 18.04 with a NVIDIA GT218 (GeForce 210 chipset). This card has a DVI and a HDMI output.

The DVI runs to an LG monitor. It runs at 1920x1080.

The HDMI runs to a new U2520D. The firmware is M3B103. I want it to run at 2560x1440, in a portrait orientation, however this option was not offered in the graphical software on my computer.

Instead, I used the EDID from the monitor to create a mode for the monitor, and used that:

xrandr --newmode "2560x1440" 241.50 2560 2608 2640 2720 1440 1443 1448 1481 +hsync -vsync

That seems to work - at least it shows the correct resolution. The monitor is correctly rotated and positioned.

After an hour or so of running, the monitor starts turning itself on and off. The LG monitor is not affected.

Even if I drop down to a lower resolution (one of the ones suggested in the UI) it still keeps turning itself on and off.

I'm using the open source nouveau drivers for the graphics card.

Are there any obvious things to try to get this fixed?