well, if you got unstable electricity power it could cause reboots
if your system is overclocked to a point where it's unstable it will reboot
if you got faulty drivers it will cause reboots in some cases
if you are dangling a wire while operating a computer loss of connection could cause reboots
are you sure you aren't getting a blue screen (popularly known as BSOD)?
Electricity is stable, no overclocking was made, drivers... I dunno, maybe... wire is ok too, no BSOD spotted. 'ts kinda freeze before reboot.