Okay, sorry about the noob comment then.
I would have thought the 6990 was v. similar to 5970 but there ya go. I think it sounds like from some of the other threads that SDK 2.4 and 11.4 driver is better with 6990 ... also maybe phoenix miner might be different with high AGGRESSION setting ...
Sorted!
SDK 2.4 give more than 95% for each GPU on 6990, while SDK 2.3 only can do no more than 65% of load....