I have similar issue with my RX 5700. The screen goes black and then it reboots. I have a suspicion that there is an issue with the power level changes in the card. What seems to help me is installing a program called radeon-profile that let's you pick a power leve. So instead of auto, i use low for when i'm not playing and high for when i'm playing games. This way there is no auto power level changes and the card seems to be stable.

It's not guaranteed to help, but you could try it out (i made screenshots of the needed steps):

https://imgur.com/a/R0FihCs

Open a terminal (press windows key and write terminal it should give you an option)

1. Write in the command: sudo nano /etc/default/grub press enter (wherever there is a sudo word in the command, you will need to provide your password). This should open a file for you.
2. It should have a line called GRUB_CMDLINE_LINUX_DEFAULT, on that line, in the qoutes, write in amdgpu.ppfeaturemask=0xffffffff then press ctrl+o to save the file, press enter, and then press ctrl+x to exit.
3. Write in the command: sudo update-grub and press enter. This should update your grub configuration with the previously provided options. After that restart the computer.
4. Write in the command: sudo add-apt-repository ppa:radeon-profile/stable, this will add a repository that has radeon-profile program in it.
5. Write in the command: sudo apt update, which will update your repositories.
6. Write in the command: sudo apt install radeon-profile, this will install the program.
7. After installing it, press window button and write in radeon this should provide you with an icon of the program, press on it. If it's not present, you can still launch it by just writing radeon in the terminal, pressing tab once should give you names of commands that start with radeon, write the one that has radeon and profile and press enter. This should open a gui for you. On the top left side of the gui, there is an option called Performance . Here you can press and choose either low, hight and automatic. Low will make you gpu run on the lowest power settings, pick this when you're browsing the internet or doing non graphic intensive tasks. High sets all frequencies to the max, so you can use this when playing games. Automatic is what's causing me problems, so you should not use that.

You should start the program every time you start your ubuntu distro (it's possible to make this automatic, but until you haven't even seen if this works for you, there is no point in doing that) and choose the low or high power option depending on what you are doing. The program also let's configure your fan profiles and undervolt the gpu, but you have to be careful there, as it does not have any hand holding, so you could accidentally fry your gpu.