Another update: I ran a memory check using Memtest 4 with no problem, neither a chip fail nor a power shutdown. Trying a new video card did not work; I still got a shutdown. Trying a pair of Ubuntu live discs gave the shutdown. Now, since I have changed the mobo, the power supply, the video card and even unplugged the HDDs in the system (I thought possibly there was bad sectors on the boot drive-unprovable because I would get shutdown before a chkdsk run would complete), the only thing left would be the CPU: the only common factor through the various iterations in the test regimen. I do have a new one around somewhere but my servicing area got moved within the shop so things are scrambled. At least if I have to get another one, they are cheap.