Desktops: Computer shuts off, buzzing noise, heat-related

Computer shuts off, buzzing noise, heat-related

Hi all. My computer is custom-built from a Tiger Direct kit -- 1 G RAM, AMD 2900 processor, 400 DDR, running XP Pro. It's a year old.

This is my problem: after running for a while (sometimes days) with no problems, it'll suddenly shut itself off and start making a buzzing noise. The buzzing only stops when I cut the power supply off at the back. This only happens when the weather warms up (we don't have central air, just window units, and the room we use as an office only has fans, although we blow in AC air from the great room and keep the room's curtains closed. I also keep a 12-inch fan trained on the innards during warm weather and have left one side open since this problem started). The guy that built the computer was here last summer working on our network when it happened one out of many times, and he theorized that the power supply was underpowered and overheating -- training the small fan directly on the open CPU was his suggestion.

The original 300 watt power supply died a couple of months ago, so I installed a 450 watt replacement, after completing a questionnaire on Tiger Direct determining how much power my CPU really needed (I purposely overestimated how much power I'd need when answering the questions, just to provide a margin of security). It had been doing fine until about a week ago, when the weather warmed up and -- you guessed it -- the computer shut itself off again and started buzzing. Again, while the computer shuts itself down, the buzzing only stops when I switch off the power supply from the back of the unit, and I have to let the computer sit for at least 15 minutes before starting it up again. Today, after a long afternoon away from home, we came in to find it buzzing away -- no telling how long it had been doing this. I left it off for nearly an hour before powering it back up.

So does anyone have any idea what's happening? It's obviously heat-related, but hubby's computer (an HP with similar specs and age) is subjected to the same conditions and never does this. (In fact, when this first started happening last year, he didn't believe me until it happened with him in the room.) I have trouble believing it's the power supply again, as this is a new one and much more powerful, actually slightly more power than I need.

Is my fan not powerful enough? Do I need some type of extra cooling system? The CPU box actually contains a chimney, which was touted as "superior cooling," but something is obviously amiss here.

Any suggestions would be more than welcomed.