If you’re running into hard reboots, it’s not just a software issue.
It’s most likely a thermal protection or an overvolt protection kicking in. Most likely causes are overheating components (maybe by dust, faulty fans etc), unstable overclock or a faulty/underpowered Power supply.
MSFS is a heavy workload, stressing out both GPU and CPU, especially with slightly older hardware. Most other games will only really stress out the GPU.
So yeh, first step would be cleaning out the PC, checking your thermals (I suggest HWINFO64) and remove/reduce any overclocks. If those are all fine, try and see if you can find a newer / higher powered PSU to test to see if that fixes your issue.