One card in rig slows down in ~30 minutes from mining start

Hello everyone!
Have problem with rig. There are three cards and one of them slows down from 700kh/s to ~300kh/s in ~30 minutes from start.

All cards has modded BIOS (voltage changed to 1075),
CARDS (3) - gigabyte r9 280x (http://www.gigabyte.com/products/product-page.aspx?pid=4845#ov),
PSU - CORSAIR AX1200i - 1200 Watt (http://www.corsair.com/en/ax1200i-digital-atx-power-supply-1200-watt-80-plus-platinum-certified-fully-modular-psu.html),
MB - ASUS F2A85-V PRO (http://www.asus.com/Motherboards/F2A85V_PRO/)

Same in Win7 x64 and in BAMT (https://litecointalk.org/index.php?topic=2924.0).

Tried many setting for cgminer - same result.
Last settings from BAMT:

Code:

{
"pools" : [
	{
	}
]
,
"intensity" : "13",
"vectors" : "1",
"worksize" : "256",
"kernel" : "scrypt",
"lookup-gap" : "2",
"thread-concurrency" : "8192",
"shaders" : "0",
"gpu-engine" : "900-1055",
"gpu-fan" : "100",
"gpu-memclock" : "1500",
"gpu-memdiff" : "0",
"gpu-powertune" : "20",
"gpu-vddc" : "0.000",
"temp-cutoff" : "90",
"temp-overheat" : "81",
"temp-target" : "75",
"api-mcast-port" : "4028",
"api-port" : "4028",
"auto-fan" : true,
"expiry" : "120",
"gpu-dyninterval": "7",
"gpu-platform" : "0",
"gpu-threads" : "2",
"log" : "5",
"no-pool-disable" : true,
"queue" : "1",
"scan-time" : "30",
"scrypt" : true,
"kernel-path" : "/usr/local/bin"
}