Random reboots can be caused by several things, heat (processor getting hot), memory errors, power supply problems, video, ...
If it is a new build, you probably need to take it down to minimum equipment. Anything you can remove for testing, try.
Information about your system would help, but making sure the video card is seated completely, as is the memory and maybe even taking out all but one stick of that.
Does you system have any red leds showing on the motherboard.
Are you overclocking yet?
Does it have the problem if you start in Safe Mode. That is how we normally limit the type of software starting up.
Edit: When you work inside the system, make sure you have turned the power off or unplugged. The board will stay hot is some areas which might cause a problem. And if you do not know, static electricity can kill a memory module very quickly, so take precautions.