Laptop oddities

I have a laptop here that's running WinXP SP2, and has been running it surprisingly well for quite some time now. however, in the last week or so, it's been turning itself off for no apparent reason when it's left idle for more than about 10 minutes or so. not even shutting down, just flicking off like a desktop that's had the power cord removed. as you can probably tell, this is a major problem when I leave it running a download or a torrent, or leave it to sit and mull over a program that it's installing.
I've already checked the obvious stuff; the power cable is in properly, the power saver settings are set to never shut down or standby without me telling it to, and I've run a full virus scan with the most updated copy of Symantec Antivirus. but still, it keeps on powering off for no reason, and I'm at my wit's end as to what to do. please help, anonymous!

Does the Events Viewer show anything strange at or near the times?

Got the newest Windows Updates? Could be an RPC or something.

WARRANTY

it's seven years old, and second-hand. I'm pretty sure it's not under warranty.

Heat issue, perhaps?
Thermal shutdown protection maybe?

power supply

What laptop brand? I had the same problem with a Cyberpower laptop that I got about a month ago. Turned out that it was a mobo problem. Everything is fine now that they replaced it.

it's a toshiba 2180CDT, and the power supply is working fine.
and it couldn't be a heat issue, because it's not running for long enough to get that hot, and it used to get MUCH hotter long before it ever had this problem