Try lowering the diff on your 6990, even try multiple threads on a lower intesity. Yes you get a slightly lower hashrate (difference on my cards is like 5-10kh/s) but you get less stales and a higher return, at-least that's what I have found anyway.
Thanks for your advise.
However, I do need the high intensity because this rig is also CPU mining XPM.
If I cut intensity below 18 the hashrate skydives, unless I stop the CPU miner.
The 6990 does 950 Kh/s on I 19, with CPU miner enabled it drops to 915.
Any lowering of intensity has a dramatic impact on the hashrate. (I 17 gives 600-650)
So, thats why I'm reluctantly pointing it at a LTC pool. High hashrate, almost no rejects, and my CPU is doing something usefull.