It's probably one of these two: The PSU burning after so much time of intensive load, or the CPU overheating WAY too much.
This is how you can know if it's CPU or PSU: After the 5 hours, just run any program that can monitor CPU temperature (there are a lot of programs, just look it up on google), and if the CPU temperature is in the 80+ range, get a new CPU cooler immediately. You don't want to fry your CPU.
If it's lower than that, it's probably the PSU, and still, get a new one immediately. These two are really important.