Spontaneous reboots are usually caused from overheating. Check your CPU heat sync and touch it make sure it's not super hot, it should only be luke warm at the most.
Reboots also are caused by faulty memory, try taking 1 stick out.
Try updating your motherboard chipset, that could be another reason.
It also could happen from a poor quality power supply or one that is going out.
Power supply or overheating are the 2 likely culprits, check both.
