I’ve been struggling off and on with an issue on my Dad’s computer the last month or so. He said that it would occasionally just shut off completely out of the blue. (He also admonished me not to spend much time on it. Unfortunately problems like this are usually the ones that take the most time to solve because they seem random and it’s hard to pin down the cause.) He runs Windows XP primarily (although we have an ubuntu install setup as a dual boot choice.) In the last year the power supply and video card have been replaced for similar behavior. Although it’s been several months since the replacement with little “misbehaving” since. One of the things I did was run a memory test, which came out fine.