Computer crashes 2 games after 20 min, but none others

[DanielDaly]

During some games such as COD WaW and Left 4 Dead, after about 20 minutes, the game will freeze up and lock with a grey color on the monitor, the sound will loop for some odd seconds, then the monitor loses signal. I have to manually turn off the computer at this point. Around the same time that this started happening, one of my USB ports ceased to work, and still does not.
Other games, such as WOW, BF1942 and many HL2 mods don't crash though.
Can anyone hint at what may be going on? I've tried Memtest86, which would simply not run past boot... I'm sure that's a bad sign. But Vista's memory test program showed no errors. I've done graphic card stress tests, mobo and cpu tests, and I can't replicate the problem. I've updated drivers for my graphics card, but nothing else.

Windows Vista Home 64-bit
Intel G33 chipset... it's an Acer AM5621 and it doesn't state what the mobo is.
Intel Q9300 (not overclocked)
NVidia 9800GT (not overclocked)
8GB's of DDR2, again the make is not specified.

 

[Lavarin]

Left 4 Dead and WaW are available Steam, some I'm assuming your using Steam right? Have you tried disabling the Steam overlay, the interface that lets you access Steam within a game itself? That's known for causing problems.

Also, if you want to really eliminate your graphic card as the culprit, is it possible for you borrow another video card and try it on your system?