That does not sound like a graphics card problem. It sounds more like heat.
Are you sure you did the yearly cleaning? See the video at http://reviews.cnet.com/4520-11319_7-6240575-1.html
After that if it's still slowing down, try CCLEANER
I have a Dell Dimension E520 with additional ram. It works pretty well
until I start photo editing with PSE5. After approx an hour of editing it develops severe constipation often requiring a reboot. Dell support
recommends purchasing a new graphics card. There are numerous ones on the market. I would prefer not having to upgrade the factory installed power supply. It's rated at max 264w. I do not do gaming, only heavy photo editing with large tiff and raw files. My OS is Windows XP. Anyone been down this road before that might make some suggestions? Thx for any advice.