I had problems with a Gigabyte motherboard and it was the PCI slot causing random shutdowns. Putting the graphics card in a different slot or using inboard graphics stopped the shutdowns. I had to replace motherboard in the end.
In your case, the 30 minutes is odd and some timer event, power saving etc must be the cause. Is there a power save setting on USB peripherals or other external devices.
As mentioned by Prazum, disconnect everything you can, take out graphics card and use inboard etc and see what happens.