My Geforce7600GS was showing signs of dead, so I ordered an HD3650 to replace it. I got it, inserted it into the AGP slot, computer started up fine. Installed the drivers. Restarted fine. Started up F.E.A.R. As soon as it finished loading and was about to go into gameplay the computer just shut off. Rebooted with a "Thread stuck in device" error. I changed drivers and tried again, same thing. The card runs fine until I do something with it. Am I going to need an RMA?
EDIT: now the computer won't post if the card is in, I replaced my old busted 7600GS and it started up fine...
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Based on that you have a decent PSU I would think that RMA would be in order.