Computer shutting down
The thing is that if that occurs while in BIOS, and it occurs at exactly 30 mins then some sort of timer is running. It won't be anything relating to temperature or power since both would be shutting down the system at a not specified and exact set of time. So it must be BIOS or any other rom type software inside a chip or controller.
Is this happening on a new motherboard since you first installed it or it started to happen after a few weeks/months?
Try unplugging everything. Remove the motherboard battery for a few minutes and reset bios. Plug in the the motherboard on the PSU (preferrably another PSU, perhaps an old one which will still have enough power to boot a mobo-ram-cpu) with only the CPU, RAM and a keyboard on Also I don't know what firmware you motherboard came with, but if it wasn't the first version then you might want to give it a try just to make sure it is not a firmware bug which got inherited from one version to another.
Taken from Toms Hardware web site