Do you get BSD (blue screen of death) at all just before it reboots?

Check your video card settings. You might find something here 1. The way to fix the Blank screen on startup,
2. GeoForce 3D setting, 3. Fix for LW interface blank spots problem

If BSD then see Limewire and Blue Screen of Death (click on link). It could be something like a ineffective fan, inappropriate heatsinks, graphic or other drivers out of date, etc.