From experience restarts are usually caused by not enough voltage to cpu/north bridge or faulty memory modules/ram especially when overclocking although some motherboards do not correctly setup the cpu by default or have a tendancy to undervolt by a little bit (sometimes just enough to cause reboots).
Also it could be a power supply fault/failure, hate to say it but hardware was not built to last forever and some times power supplys start to diminish(is that the right word) in quality over time .
Does this crashing/reboot happen with any other program/s, software or games etc?