Sorry to report I switched the 6990 rig over to a LTC pool.
Reason : rejects all over the place.
The 7970 rig is still happy moooo-ing away however.
As I mentioned before, the 6990 on I 19 setting does not like the low diff coins.
The 7970's on I 13 dont care about it.
Same mobo/os/miner etc.
It has nothing to do with the pool.
Perhaps I'd better sell the 6990's and buy back 7970 instead....
Yeah, this has to do with the intensity value. Dropping down to i18 on a 7950 helps some, at the loss of hashrate, of course. Perhaps not a bad compromise, it's what I used back when I was mining DGC.
Since the 7970's for whatever reason like i13 as a sweet spot, as you say, they do quite a bit better.