With videocards, you might not have enoughh airflow around it.
Which makes perfect sense, as you use it, the air around it (which the fan is using the cool the heatink) is getting hotter and hotter. This heat buildup should be gradual (which it is), and should be easily fixed by getting a fan or something to provide more airflow around that area specifically.
If you can have a fan blowing directly into that area, it should lower your temps dramatically.
Also, it doesn't make sense, your profile says you've got a pretty good setup, I'm assuming this is another pc that you're having trouble with? Was just looking for information on the case, to see if you can put a fan near the HDDs, which may help, although it might be too far away to actually move air.
If thats not possible, PCI slot fans may be the way to go.