Try different engines and memclocks on the 7970's.  I've got 2 identical rigs with 4 7970's and powered risers and they each like different cgminer settings. Strange since they are literally the same machine as far as components go.   Also I've found that plugging the monitor into different cards and rebooting will change or improve the hashrate.  It's all black magic with 7970s, especially when you're running 4 of them.   I don't use dummy plugs on any of the cards that the monitor isn't connected to. 

How much RAM does your system have in it? Do you have at least 4GB on the board?