No blue screens, no stack dump just a reboot.
To be absolutely sure that it none of that is actually happening, have you checked your Windows error logs?
Sometimes it happens so fast that you cannot see it happen.

If there's no errors in your logs I am going to have to side with the people saying that you may have a PSU problem.
I don't think there's a heat problem because most modern mainboards will shut the computer down completely if that is the case.