About six weeks ago my computer started shutting down suddenly. Power just cut out. Its a Athlon 64X2 4200+, 4 GB RAM, 4850 video card, Vista 64.

I tried to reproduce the problem. I wasn't using it for anything taxing, no gaming or anything, but when I started playing some HD mkv files on it, that caused it to shut off. So I cleaned out all my fans and CPU cooler. Installed fan and temperature monitoring problems. Sure enough CPU would get hot, to about 47 C, and then shut off.

I put in a new power supply, as the old one was filled with dust, I figured maybe that's overheating. That initially seemed to solve the problem, but then it started shutting off again.

So I reinstalled my OS, everything clean, just to make sure its not some bug or virus. Now what happens is interesting. When I turn it on in the morning, it starts to shut off almost immediately, sometimes barely after getting out of BIOS. Then as I keep turning it on again, it gets further and further in the boot process each time. Then at some point after about 20 minutes it will just start working. I'll play HD video on it, no problem. Its staying pretty cool, under 45, and it almost never shuts off on its own again, until the next morning.

Now I'm not too attached to this computer, as it survived a burned out motherboard, so that may be the culprit, but I'm really curious why it would keep shutting off when I start booting it up, but after it gets "warmed up" it will run for 10-20 hours.