-Check your Vcore, maybe you should lower it, or set it higher
-Maybe you have a problem with your memory timings or
-maybe your mainboard has a wrong setting for the max temperature of your CPU, and thus rebooting your system.
-Maybe you have a PSU problem?
Oh dear, sounds like it could be more serious than I thought. I'm not sure what memory timings or Vcore mean, and now I've uninstalled F&H, CPU temp is back to 42ºC. Seeing as the problem only occurred when I was running F&H, the simple solution seems to be to NOT run F&H, so I'll leave it for now until I get the new case up and running.
Thanks for the feedback.
I thought maybe you changed the memory timings and Vcore, but if you don't know what they are, you wouldn't have changed them. Vcore is the current voltage of the CPU. If it's set to low your system might reboot if the CPU is under full load. (the CPU hasn't enough energy in that case) Maybe your mainboard has set it to low, or your PSU is faulty. I think you can check it with motherboard monitor 5.
I had some trouble with the Vcore also, because the CPU was undervolted. When I started F@H the system would crash, sometimes right away, or sometimes after half an hour.
The Vcore should be between 1.47 and 1.53 volts. If it's lower you can change that in the bios.
If that doesn't help you should check your memory like Lenny said with MemTest and if that doesn't help I think it's your power supply.
You should fix your problem, you don't want your PC to become unstable and crash and lose all your data.