Give some more details :-
Driver version ?
Mining software ?
6990 fans seem to be toggling due to over-use . You can oil the fans , it seems to help .
Win8 Amd 14.2 catalyst
The device is not detected properly in device manager. I am having TX 750 PSU. Fans are toggling to 100% then 10% for 5 seconds and this continue. CGMIner not detecting 6990 but 6850 only.
Thanks anyway