Re: power trouble
Ok hmmm, Just feel I should suggest something as I was facing similar problems a while back with a previous system. This might be textbook, but a computer shutting itself off, in my case, was due to the powersupply being too weak, as in there is not enough power to support the components.
As your problem seems to be focused around power I suggest you list your powersupply model and wattage here to see if anybody else can offer advice.
Hope this helps.
My Setup -
AMD 64 3200 +, 256 mb Geforce 6600, 1GB PC3200 DDR400 RAM, 160GB SATA 7200RPM HDD