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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.

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It certainly sounds like something is scanning your system on a regular basis. Have you checked for viruses & other malware? – ChrisF Feb 25 '10 at 12:19
Hmm. I've got a virus scanner (as mentioned below that I tried disabling it), which claims to be up-to-date and runs checks every couple of days, and I've seen this behaviour for a couple of months. But I may as well run a full scan just in case. – AlexC Feb 26 '10 at 10:56
up vote 1 down vote accepted

Found the answer. It was inspired by finding this thread: which suggests a temperature issue, or a problem with the cooling of the graphics card. Sure enough, the fan on the graphics card had got stuck and wasn't turning. Presumably the card was periodically overheating, slowing massively, then resuming processing once it cooled enough.

I was fortunately able to re-enable the fan, and the issue's gone away. Thanks to ChrisF for the suggestions and comments!

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Do you have an anti-virus scanner running and is it either set to continuous scan or to scan on a very short time scale?

It could be that the application is updating files which is triggering the anti-virus software.

Try disabling it and run the application then. If that cures it, then look at the anti-virus options to either disable the continuous scan, increase the time between scans or omit certain directories from the scan.

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Very good suggestion. I tried disabling the virus scanner (and my wireless internet connection), but no, it still happens, sadly. – AlexC Feb 25 '10 at 11:34
@AlexC - is there any consistency to what you're doing in the (ah) application when the slow down occurs? e.g. viewing a particularly complex scene? – ChrisF Feb 25 '10 at 11:52
No, no consistency. Repeating the exact same sequence of commands will have the slowdown kick in, or not, at different points each time. The app or game continues to be unresponsive even if I pause processing, though I've noticed that switching it to use a different screen resolution does break it out of the slowdown. The problem does keep happening across a variety of different apps and games, but only when something is using DirectX-style graphics. Apps or games running in their own windows don't trigger the slowdown. – AlexC Feb 26 '10 at 10:53

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