EDIT: I am using Asus R9 280x DC2 TOP version 1, I am getting around 780kh/s for each card.
May I ask your cgminer configuration, please?
+1
780kh/s seems very high
Not that special. Seen ppl doing 800 on a 280x, but those are hard to find...
I get 773kh/s on my 2 280x dual sapphire's @1110/1800 (didn't up the voltage though)
How did you do that? I have 4 280x dual sapphires and whenever I turn the clocks up that high, they crash. I can't go over 1050/1500, and that gets me 730 khs.
My rigs were doing great, but I tried switching to the useast server and my rigs started doing bad... lost 1,500 khs... I tried switching back and it's not doing much better.
Just used cgminer to up the core and mem speed, is all. You sure your psu can handle that many on those clocks? Maybe it's 12V line aren't stable enough when you draw that much power. I just bought them a week ago, so maybe I have a new revision? But tbh a mate of mine has the stock sapphire and can run his ram @ 1800 too.
You could try upping the voltage, I didn't need to, but every chip is different.
-edit- Oh, are those plugged in x16 slots?