Are you using Afterburner?  You're monitoring temp, but what about CPU useage?  I have 4 7950s, and I have a similar problem.  They all run around 73 Celsius, but one consistently runs at 79-80.  Ive noticed via the CPU usage all 3 cards running at 73 C   jump between 99% useage to 64% usage.  They just toggle back and forth all day.  I was concerned at first, but it keeps the temp down, and I'm still get a good hash rate.  My problem card (4rth card) runs at 98 - 99% max useage all the time, thus, the temp stays pretty high.  Same setting as the other 3 card, but it won't toggle