I would take it out and clean out the dust with some air to make sure its clean.
If that doesnt work, take the side off your case. If that works, you have bad case airflow.
If that doesn't work, you may want to take off the cooler, clean the GPU with a q-tip and alcohol then get some paste and reapply. I've seen this issue with older cards before and the factory paste sometimes squeezed out or gets really hard and doesn't transfer heat well anymore.