The 480 series is right about three years old assuming you jumped in at product release. This is typically the point when zinc oxide paste (thermal compound) frequently used by vendors has dried into dust. You might need to remove the heatsink and apply a fresh layer.
This of course presumes that nothing else has changed. No new case, no new motherboard, no new expansion slot arrangement, no new CPU, no new expansion card, and no new changes to the airflow within the case (that includes CPU HSF changes).
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