Most likely you are having bad connectivity with AGP slot. If there is some oxidation on the bare metal connectors, best thing to do is to take some fine sandpaper and try to polish them out. Don't push too hard and don't use rough sandpaper. Most VGA cards have gold-plated surface, and you don't want to remove that protective layer.
Chaky
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Socket is a different story.. I wouldn't advise you to try that. You would risk some serious damage (mechanical).
Chaky
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I think you've got all the swap around angles covered so that you can eventually narrow it down to either the card or the mobo. If it's not the card, I wouldn't p*ss around with the mobo if you're not happy with fine soldering. It could well be a dry joint under the AGP slot as Chaky has suggested.
Suspishio
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