Hi guys, Im in the process of setting up my rig and getting more cards as I go

I started with 2 and have just bought another 3, all to run through one motherboard. However, I am having trouble with just adding in the 3rd at the moment.

I can crunch away all night long with 2 graphics cards, but as soon as cgm boots with the 3rd card in, the system shuts off instantly. Doesnt even restart like I told the bios to.

My rig specs:
AsRock FM2A85X Extreme6 mobo (has 3 PCIEX 16 slots, however I can see that one is 8x and the 3rd one is 4x)
A4-4000 APU
8GB Corsair ram
Crucial AX1200i PSU
Gigabyte R9 280X (x3) - modded bios to run at 1150mv as opposed to the default 1200. I have tried default and the same thing happens. 2 cards run fine with 1150.
Xubuntu 13.04 / cgm 3.7.2

Ive built an aluminium open air case and the GPU's never go above 80C - Ive even run a wire from the top bar (where the graphics cards screw in) to the bottom, ensuring good grounding. Ive got dummy plugs in all cards. Im using powered PCIE 16-16x riser cables plugged in to different molex connectors off the PSU. Got a couple 1 - 16x risers coming soon hopefully so cant try the 1x slots yet.

Thats about all I can think of that would be of any use to anyone who is kind enough to lend assistance but if you need more, then please ask!

Any ideas guys?

Thanks