The general reasons for system instability are problems caused by drivers and software installed or due to incompatible settings in either the BIOS or some firmware you might have installed for some hardware.
As far as your problem is concerned, most probably it is a BIOS, firmware or APCI (Advanced Configuration and Power Interface) related problem. You should be able to get some hints from the BSOD screen (the blue screen which appears before the shutdown).
You will have to refer to the motherboard manual and set the correct values for the BIOS settings, like the voltage settings, APCI settings etc. You could try loading the default BIOS settings as a starting.
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