Hi all,

I am new to this game (but luving it already), and have made some good progress in putting a rig together, however I have a problem that I just cant seem to solve ... any suggestions would be most welcome.

I have a single motherboard MSI-B85M-G43 with 2GB RAM and Pentium G3220 CPU. The MB has 2 PCIe x 16 and 2 PCIe x 1 slots and I am running windows 7 64 bit and the latest AMD drivers dowbnloaded from the net. I use CGminer 3.7.2

My intention was to populate the MB with 4 Saphire R9 280x TOXIC GPUS.

The first GPU gave a steady 750 Kh/s
Adding a second GPU gave an additional 750 Kh/s for a total of 1500 Kh/s
Then came my problem, Adding a third GPU causes the rates on all three cards to drop to approx 580 Kh/s !!!!

I have tried everything I can think of including:
- Powered and un-powered risers
- Swapping PCI-e slots
- re-tuning CGminer settings
- Changing power supply (I have corsair 1050 Watt)

my miner settings are:

Intensity 13
Engine clock 1100
Memory 1500
Thread concurrency 8128
Worksize 256

No matter what I try I get the same results - the rig seems not to want to go above 580 per card with three in together ! 
Has anyone else had or heard of this sort of issue and find a reason for it ?