Hi similar setup to mine.
Set the following:
In the ati control center -> performance -> graphics overdriver
AMD Overdrive: enable (ticked)
Power Control Settings: +20%
Enable Manual Fan control: enable (ticked)
set fan speed to 80%
Click apply.
I suggest you use cgminer.. seems to be pretty well optimized for 6990/ 6970/ 6950
http://forum.bitcoin.org/index.php?topic=28402.0start cgminer with a batch file (runcg.bat) like this:
cgminer.exe -o http://ozco.in:8332 -u username.1 -p password -I 8