First guess is heat. If a computer crashes when it's working hard, but not when it's sitting idle, then it is often heat that is the issue. Easy test ... remove the side cover and fire up Fortnite. If it doesn't crash, then heat was the most likely issue.
2nd most likely issue is memory. A bad memory stick can cause all kinds of issues. You can test this with windows memory test.
3rd most likely is a power issue. Looking at your setup, power shouldn't be an issue, but things happen.
From there it gets more difficult to guess ... we would need to know more.