Last month I re-cased my computer. It's a 1.3 GHz Athlon from HP. Never liked the case, so I got one from surplus and did a motherboard transplant. The 'new' computer worked fine for me, but my children started complaining that their games were crashing. I watched a few times and the thing would just do a cold restart whenever the graphics card got busy. I'm running an ATI Radeon 9250.
I thought it might be a heat problem. The old case had a cowling that tied the main case fan directly to the CPU. It didn't fit the new case, so I just tossed the cowling out. The new case uses 80 mm fans, where the old one had a 120 mm fan. I had just changed the old fan for one with a green LED, just 'cause I like the glow in the background. Anyway I put the old fan into the second fan space, but it still crashed.
At this point I started thinking harder. Eventually it came to me, maybe this isn't a heat issue. Maybe it's a power thing. I kept the older power supply that came with the case. It was 200 watts, while the HP supply was only 150. I figured I was doing a good thing with those 50 extra watts. But it was an older supply, and maybe it wasn't as good at regulating the power. Rather like Apollo 13, when they only had so much power to restart the command module. Maybe when the GPU fired up I started getting 'BUS UNDERVOLTAGE' and that's what was triggering the cold restarts.
The ATX power supply has to provide a variety of voltages at different current levels. I compared the labeled capacities between the HP supply and the older supply. The older was rated for higher current on all the listed ports except the +5SB, which I guess is the USB serial bus voltage. I put the HP supply back in the computer hoping that the older supply just wasn't performing to spec.

Looks like that solved the problems. The kid's games run just fine so far. Maybe I'll open the older PSU and see if there are any obvious problems. Years ago I got the training, so when they say 'no user serviceable parts inside' they're not talking about me. I've heard of people re-using computer power supplies for workbench or radio power supplies. Maybe I'll give that a try. It would be interesting to see just what the old box can do. I don't usually work with whole amperes at a time.