Could be a few things. Are the cards right next to each other? Often the lower card's cooling is sacrificed by being butted up against the card above. Lower voltage almost always lowers temps, so undervolting should help. Generally speaking, you should be able to undervolt a bit and be able to leave the clocks at stock speed, but it is card dependent. Lastly, the 7970's have been out for a while, it's possible that the TIM has dried out and needs replacement. I had a Gigabyte 7970 that started overheating and didn't get any better until I replaced the TIM and thermal tape. Again, depending on the card, it can be a pretty easy job or a PITA.