Are you running powered risers? I have 5 Dual-x's and I'm getting around ~730 kh/s. You're gonna have to tweak the settings around to what the cards like. I'd try putting the voltages around 1.1, core clock @ 1040, and I found out my Dual-x's like thread concurrency 11200. After you get a stable baseline for the cards, that's when you can start tweaking for better performance. 10 more hashrate is going to produce more if you're getting rejects and errors
I'm running without risers straight off the board for now.
As for the problems, by lowering the culprit card clock to 1060 I've now been getting a consistent hashrate of 740 Kh/s, so next step is probably to drop the core clock on the other cards to see if that has any effect on their Kh/s, if not and it stays stable I'll probably run them all on 1060 as it seems to work.
If 1060 works, I'll probably try 1050 too, and eventually maybe even 1040 as you suggested.
As for the invalids I always get them when the rig has been started, and they eventually stop coming around, now almost 3 hours after my last update of the earlier post the card that had 2/6 invalids has 3/60.
Having to tweak the cards individually is kind of a hassle, one would expect the manufacturer to slim up their margin of error a bit when they ship cards at 1.225 - 1.134v, the power consumption difference between the two is enormous.