System freezes when mining with more then one GPU

Hello fellows!
I hope to move this to the support forum when i'm "trusted"^^
I have a severe Problem with mining.
I use CGminer and whenever i use both of my R9 280X in any way at the same it will mine for 1-5 seconds unti my system freezse completly. I know the GPUs aren't working while this happens because i only use 150W while it is frozen.
I'm using a brand new 64-bit win7 ultimate with ATI Catalyst Suite 13.12 and cgminer-3.7.2-windows.

I'm starting just by double clicking on the miner, since i have it all configured in the cgminer.conf. I get the same error, when i use a *.bat with the configs in it.

My cgminer.conf

Code:

{
"pools" : [
	{
		"url" : "http://coinotron.com:3334",
		"user" : "Valansch.3",
		"pass" : "3"
	},
	{
		"url" : "http://coinotron.com:3334",
		"user" : "Valansch.4",
		"pass" : "4"
	}
]
,
"scrypt" : true,
"intensity" : "13",
"vectors" : "1",
"worksize" : "256",
"thread-concurrency" : "8192",
"shaders" : "2048",
"api-port" : "4028",
"gpu-threads" : "2"
}

When i add a "device" : "0" or "device" : "1" everything works fine with just the one GPU in use, i also tried running 2 instances of cgminer--> no sucess

My System + Hardware:

Code:

Windows 7 Ultimate 64 Bit
CPU: AMD Athlon II X2 250 Processor
Mobo: AsRock 990FX Extreme3 AMD 990FX Dual Channel
PSU: 850 Watt MS-Tech Value-Edition retail Non-Modular
Ram: 1x 2048 MB Kinston Value Ram DDR3-1600
GPU 0: Powercolor Radeon R9 280X (in Slot 0)
GPU 1: Sapphire Radeon R9 280X (in Slot 4)

I would really appreciate any help!

PS: I'm sorry if there are any grammar or spelling mistakes. It's not my first language have mercy

But if you finde a mistake feel free to shoot it!