When the computer reboots during reboot, what happens exactly? Do the hard drives/fans stop spinning? Do the lights on the motherboard turn off and back on? If everything turns itself off then back on again, it most likely is a power related issue. However if the motherboard simply "re-initiates boot", then it is most likely an issue with the motherboard or another hardware component. I would specify what component but to be completely honest any component could be causing the issue. For example once i had my sata power cables plugged in upside down (don't ask how, it just happened) and my system would refuse to post. Computers are strange things aren't they not?