Still sounds like an overheating problem. Is the CPU heatsink OK? Is the case ventilation OK? Does the motherboard have a temp. sensor? Perhaps you can change the temp. at which it is set to shut things down.

Try temperature regulation as people have said. That can make a big difference.

If that is not the problem, I wonder if your problem could be power related in spite of APC. If nothing else works you might try a battery back up that includes a voltage regulator. That you had two transient outages in a short time your battery backup took care of is not a good sign Unless the one you tried included voltage regulation?

dcp, I don't have a clue how to check on either the heatsink or the possibility of a motherboard temp. sensor. I did ask the Best Buy people to check on it, and they said it was fine. (Of course, there's always the possibility that they didn't do anything to it at all and just took my money; how would I even know?) The case ventilation seems OK; nothing's blocking it and I feel air moving, anyway.

But if it were overheating, wouldn't it have done this at Best Buy or my old place, too?

Still sounds like an overheating problem. Is the CPU heatsink OK?

I agree. My gut tells me the CPU is faulty.

TypoBoy, my APC does include voltage regulation. Backup battery will intervene if AC utility voltage goes above 136 V, or below 90 V. (I can change these numbers.) Every time I check it, it says the current voltage is 115 V.

The last transfer to battery was caused by electrical noise, which lasted under 5 seconds; the one previous was a blackout, also under 5 sec; both I believe were related to thunderstorms.

how to check on either the heatsink or the possibility of a motherboard temp. sensor.

Checking the heatsink requires opening the case. The temp. sensor and contol setting in my (five year old) machine can be accessed through SETUP -- before Windows loads. I can get to SETUP by holding DEL during boot-up, but that may not be how it works for your machine.

But if it were overheating, wouldn't it have done this at Best Buy or my old place, too?

Maybe, maybe not. There are too many "it depends" variables. Was it on a cool counter? Was it kept upright? On its side? Things like that, and you have no way of knowing.

Sorry to post & run, but gotta get Dad to the airport....

Eddie, can you or your gut tell me how I, who know nothing, can find out if something's wrong with the CPU? Maybe I just need to get a private geek in here to look at it. I'm not taking the blasted thing back to Best Buy; they've had their chance.

Sometimes when it restarts, it tells me it needs to check the disk, but it never says anything's wrong with it after it does the scan.

Turn off the computer andopen the case. Make sure every single connector and cable is firmly in place as well.

After that, leave the case open and turn it on. Check to see that every fan, including the one in the power supply is spinning.

Then shut it down and put the cover on and run it for a while.

If it still turns off randomly, try running it with the cover off, that would keep it cooler. maybe have a fan pointed at it.

if it's a cooling issue, the mysterious shutdown shouldn't come back. If it does, it is possibly a failing CPU chip/card.

I will do that. Thank you.

Actually on modern case designs running them with the case open actually makes them run hotter. The fan systems are specifically designed for airflow across components to pull the heat away.