that is the temp from a white paper spec on a reference chip! the fact is in real use these cards begin to thermal throttle well bellow that. as reported literally hundreds of times in these forums. the 580s will throttle around 72 and the 590's seem to do a bit better around 78.
Setting a the power limit to max and using a custom fan curve will help a lot. if not you may have a bad card and you may wish to talk with your cards maker for advice or an RMA.
I will post a fan curve for you to model after and set the power limit the same. Don't use the speed settings!