Ok, sorry for the late reply... it has nothing to do with the components fortunally and the temps seems to be just more than fine. So its not a hardware problem. Lately fortnite its giving to many players a headache and me either. The game seems bad optimizated as f***
But lets try to see what we can do.
First, Check your Drivers are up to date and if so i would anyway DDU (Display Driver uninstaller) and remove all Drivers installed and clean install fresh ones downloaded by Geforce Experience panel.
Also if you are playing on a 144 hz monitor, in the fortnite options use V-Sync and cap the Frames to 144 or 146 (in case you can customize it). If you cant see this options, customize this via Geforce Experience game optimizer and Play the game on Full screen.
In case you are player on a 60hz monitor just cap to 60 with V-sync.