New video card freezes Mac G5

[Judith Rom]

I have a Maintosh G5 and the video card needed replacing because the screen was losing its image and "dissolving". I purchased a refurbished Radeon 9800 Pro 128 MB video card. Since then the computer works alright for about 10 minutes and then freezes. It will freeze whether I am working on a program or not. The only thing I can do is shut it off with the power key. Could it be a faulty video card or the wrong kind, and if so, should I purchase another kind? I don't use video games or anything else similar. I only need a decent screen. Thank you very much.