You need to improve the airflow in your case, are the two cards right next to each other or is there a space in between? If there isnt a space the top one might be getting staved for air in which case you need to either space them out a bit or improve your case airflow so that the ambient temperature is lower.
You might as well tell the forum , your build.
What m/b ?
We can take a look at the layout. What model of 5970 's ?
When you attempt to game, does the top card fan get aggressive and keep temps under , 90 ? I'd guess no, but maybe just at idle the top fan chooses not to spin fast ? What % does the fan on the card 72 at idle spin at now ?