I have a 20" iMac running OS 10.6.8 with a ATI Radeon HD 2600 Pro card, That I rarely shut down completely. For the last year, or so, it has at random times instantly, gone blank in one of several ways. Sometimes it is a simi-blueblack screen with light coming from behind around the edges, and sometimes it's a pinstripe black white image, or even one that is slightly monochromatic. If I restart then the screen returns to normal, and although I would have to say it's happening slightly more frequently, the increase is very slight. If I'm on my screen for mare that a couple hours, the chance that it will happen is pretty good.
My question is, is this the video card going bad? If not what else could it be, and if so, how much will I need to expect to spend to replace it? Is that something I could do myself? If I have to replace it what card should I use?
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What motherboard do you have?
There's a decent chance that it's the video card. My father had an iMac of similar vintage where the video card went out. It could also just be a matter of the card overheating. I'd see if you can clean any dust out of the vents of your computer if you're looking for a solution you can do yourself.
As for replacing the card yourself... Apple does not use normal discrete cards in the iMac. You will not be able to replace it with readily available parts if it can be replaced at all. I haven't done any reading on the subject in years but it is either a small form factor custom card or integrated as part of the motherboard. Regardless, even if it were a standard PCI-E card (which I can guarantee you it isn't) you'd need a special "Apple" version of the card, for example if you were using it in a Mac Pro.