I hadn't replied because I was waiting for it to act up again. It's been very stable.

One bit of information I have left out is that I was getting some occasional motherboard utility warnings/beeps over the past year or so about some fan spinning too slowly, but every time I looked at the fan speed it was fine, and I've never seen a high CPU temperature on it. Perhaps there is an intermittent cooling issue, though. (While I still haven't opened the case after replacing the UPS batteries, the PC remains out from under the desk and closer to an air register. Perhaps that additional cooling is masking the problem.)

I'll keep an eye on it and report back if I have any more information.

Thanks again.

Cheers,
Scott.