You cannot run 7970s with SDK 2.5; if it's working with that combination it's only because cgminer has cached the .bin files.

I had a similar problem in widows 7 after adding a fifth gpu, 4 gpus ran great, 5 sent cgminer into a tail spin with the hash rates, but worked fine under ubuntu, I figured out it though I had to set the gpu threads to 1 then got full steady hash rates on all 5 gpus

I could not get the 4th to work until I increased the PCIE latency in the bios from 64 to 96.