"Temp3 -128C: ( with 0 rpm fan speed )
I have just gone through the exact same thing here...faulty old motherboard and failing power supply were the culprits. Bought new power supply which allowed me to at least run this old board in 2D mode, my new motherboard will be shipped to me tomorrow!
By the sounds of it you are having the same sort of issue here...my system would just shut itself off for no reason while idling in Windows ( nothing running ). I would suspect the motherboard first plus possibly your power supply dying...
Do you have another psu to test with?
How old is your motherboard? power supply??
Most likely it is one or the other failing here, or both as has happened in my case...