There are a few reasons why Fortnite keeps crashing on a PC. They could be temperature, power, overclocks, drivers, or something else entirely.
The easiest way to find out is to play another game for an hour or stress test the GPU. Play something graphically intensive like Cinebench or game for at least an hour in one sitting and see if it also crashes.
Make sure your GPU driver is up to date and try running the game as administrator.
Another good stress test app is AIDA64
AIDA64 is a very comprehensive full system diagnostic utility typically aimed at engineers, IT professionals, and systems enthusiasts. Among other tools, one of the best tools AIDA64 offers is the System Stability Test which has the ability to stress everything from the CPU to RAM to the GPU.
What’s great about AIDA64 however, is that it will tell you if the CPU begins throttling, or slowing itself down when the temperatures are too high. If the CPU throttles, then it’s a good idea to check that the CPU cooler is properly installed, purchase a new cooler, or if you’ve been overclocking, it may be time to reduce the overclock.
https://www.aida64.com/downloads