I'm guessing that this is the mac pro?
That's very odd, but I doubt it's your video card if it's locking up like that. It could be a whole bunch of problems. Corrupted file tables, screwed up permissions, or even a hardware fault.
What you might want to do, is take a look at some system diagnostic tools, such as Techtool, or use Apple's own hardware diagnostic utilities from their website.
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