As David suggested, you need a new video card. Most likely, the fan is going and the card is overheating. You can test that by turning off the pc, taking the side panel off the pc's cabinet, powering up and seeing if things work better, even for a short while.

You can get a decent card at a good price and it's as easy as removing the old one and adding the new one. The only thing you have to worry about is whether your mother board supports AGP, PCI, or PCI-Express (it will support one of the three).

Telling us the age of your pc and other specifics such as installed RAM, version of Windows, etc, will help narrow down what you need.

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