Hmm the only thing that I can think of is a faulty GFX card.
Try putting the old card back in to see if it works. If that works without a problem then the the new card may be faulty.
Perhaps getting in contact with nVidia to see if there are any compatability issues, it might just be incompatible with other hardware you have.
If the old card gives you the same problems as the new card it is possible the PCI-E slot has been slightly damaged.
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