Intermittent massive graphics slowdown (25fps->1fps)

I'm seeing the following frustrating behaviour with my home PC (running Windows XP Home; dual 2GHz CPU, 2 GB RAM, NVIDIA GeForce 8400 GS graphics card).
I'll be doing some fullscreen application – usually a game – which has been running at a perfectly respectable frame rate for a while, perhaps 5-10 minutes.

Then, with no visible cause, it'll suddenly slow down massively, displaying only 1-2 frame a second, and holding up processing correspondingly. As you can imagine, this makes most things pretty much unusable or unplayable.

If I do persist with trying to use it, then after 5 or 10 minutes, it'll suddenly recover and return to full speed. But after another 10 minutes or so, it'll do the slowdown again.

This happens with several different games, all fairly recent ones. It doesn't happen when running games in browsers, even if they hit performance issues. Nor does it happen on video playback.
Forcing the app or game to change screen resolution does fix the issue, but only temporarily. After another 5-10 minutes the framerate will drop again.

I've tried:
*) Updating the drivers for my monitor and graphics card
*) Running one such game in windowed mode rather than fullscreen – to my surprise, the issue still occurred there
*) Disabling my virus scanner and wireless internet connection
*) Rebooting immediately before starting the app or game
but none of them prevent the issue from happening.

Does anyone have any suggestions for what might be the cause, and/or how I could fix or work around it?

My apologies if this is too "gaming-related". I think it's a graphics or graphics-card issue and not specific to games, but if you want to close the question as too gaming-y I'll understand.