I've created a 2nd miner, my original being a set of 3 sapphire 270xs. I ended up getting that one running with a pretty decent hash rate for the cards I was using without a to many problems.

I enjoyed setting up and running that miner very much So I figured I'd have a go at creating another.

Here's my hardware.

GPUs               3x  R9 280X, R9280X-DC2-3GD5
Motherboard    ASRock 970 Extreme4
Processor        AMD Sempron 145
Memory          Kingston Hyper X Blu 4 GB 1600MHz DDR3
PSU                Seasonic 1300w Platinum PSU

Heres my problem.

I'm getting a hash of about 660kh/s each with two of the cards connected through crossfire, but then the third card which isn't connected through crossfire is getting rates at about 740kh/s. When I remove the crossfire they go down about 20kh/s.

I don't understand why they are doing this. I was expecting rates at 760kh/s plus from each of the cards, and why they are going down after I remove crossfire is beyond me.

I am using GUIMiner Scrypt alpha
-I 13 -g 2 -w 256 --thread-concurrency 8192

I really was expecting more hash and whatever changes I make to the setting only seems to lower my hash rate. Any ideas why this may be happening?

If anyone has any thoughts, thank you very much in advance. Really wanna get this going properly!!