From the FAQ
Q: Which AMD SDK is the best for cgminer?
A: At the moment, versions 2.4 and 2.5 work the best for R5xxx and R6xxx GPUS.
SDK 2.6 or 2.7 works best for R7xxx. SDK 2.8 is known to have many problems.
If you are need to use the 2.6+ SDK or R7xxx or later, the phatk kernel will
perform poorly, while the diablo or my custom modified poclbm kernel are
optimised for it.
Q: Which AMD driver is the best?
A: Unfortunately AMD has a history of having quite a few releases with issues
when it comes to mining, either in terms of breaking mining, increasing CPU
usage or very low hashrates. Only experimentation can tell you for sure, but
some good releases were 11.6, 11.12, 12.4 and 12.8. Note that older cards may
not work with the newer drivers.
That was my last option to reinstall all the drivers/sdk. Thanks for helping, I will post here results, maybe more come with similar problem.
I noticed a big difference switching to the poclbm kernel. It's not used by default. I'm running two 7850s with Catalyst 13.4 and either SDK 2.6 or 2.7, (I can't remember which). One of my cards always runs hotter than the other. Initially it was so much hotter that CGMiner would throttle back the clock from 1050 to 860, but now they're running pretty evenly matched at full speed. (This is all on 3.1.1 by the way, there's not much if any GPU improvements in 3.2.1 from what I understand).