If the GPUs are on the motherboard, it can be high temperature of other component besides the GPU core itself, the vrm module, the ram, or even on the motherboard. Those can be at 100+ Cº by the time the core reaches 80ºC. Get speedfan and Gpu-Z softwares and see if the sensors report abnormally high temperatures (or abnormal voltages).

Use an household fan and point it to the rig. The rig is open-air, right?

Is it always the same card that becomes sick, or is it any card on a particular PCI-e slot? Does it happen when only 1 card is plugged?

Are you using risers?

Hashing differences within the same brand and model, seem to have to do with video ram. Vram from different suppliers, different specs, high end at launch and for reviews then cheap out or use more conservative settings, bios revisions, timings programmed, etc.... Some cards work best at a fixed ram clock, e.g. 1500 Mhz, others work better if you OC the ram as much as possible.

When buying a card at a store, and if it's visible, choose cards with Hynix or Samsung ram. Some Elpida may be good too, but I believe it's the "cheap" option.

Then there's a thread at litecointalk forums, where for a small fee or donation, someone mods your bios with better ram timings and therefore higher hash rate. Finally, there are optimized .bins out there, with unrolled loops and some hardcoded constants that skip redundant calculations and therefore give a bit better hashrate too.

However there are other reasons why someone can get 730+ and others just 700 with completely identical cards. Posting their best hash at night, living in the cold North and someone else not able to get them in the South during the day. Another is the coin and the pool where you are mining. The motherboard, the PSU, the system ram, all combined can also have influence (e.g. 2 - 4%) in hashrate. Likewise, the addon software the rig has, specially if the CPU is single-core and cgminer isn't running on high priority

Edit: the bit about Hynix and Elpida is innacurate. There are 2 kinds of Hynix calibration actually, a good and a lousy one. https://litecointalk.org/index.php?topic=12369.0

Both are Sapphires. The 270's come with Battlefield 4, so I'd get 5 BF4 coupons. 

I expect 700 Kh/s out of the 280x, but the 270s may be lemons with slow, high latency vram. I'd get 400 Kh/s instead of 470 like the Gigabyte R9-270 OC offers.

Is this a good deal in the first place considering the upfront cost, power cost in E.U. and the incoming ASICs, or should I forget about it?

6950 unlocked: 250 Kh/s

"intensity" : "20",
"thread-concurrency" : "7680",
"gpu-engine" : "900",
"gpu-memclock" : "1500",
"gpu-threads" : "1",


Gigabyte R9-270 OC: 215 Kh/s
"intensity" : "20",
"thread-concurrency" : "14208",
"gpu-engine" : "1100",
"gpu-memclock" : "1500",
"gpu-threads" : "1",

Yeah, listed $450 at Corsair website. Ouch!

Bronze, silver, gold, platinum has to do with power efficiency, not long term stability. For long term stability, you need to pick good brands and not push the PSU to the limits. Pull only up to 80% of the rated power out of the PSU at 24/7 operation, and even less in the Summer

Perhaps a XFX 850W or some Corsair 750W, or preferably Seasonic

About single vs 2 PSUs...

The EVGA Supernova 1300W is a not-too-expensive PSU that you may find interesting.

http://www.newegg.com/Product/Product.aspx?Item=N82E16817438011

That's $240, I don't know the prices in the UK

-I 20 -g 1 --gpu-memclock 1500 --gpu-engine 1100 --thread-concurrency 14208

gives me ~215 Kh/s

Nice video! Just to say that one can get 470 Kh/s out of the Gigabyte R9-270 OC with
"thread-concurrency" : "15232",
"gpu-engine" : "1100",
"gpu-memclock" : "1500",
"gpu-threads" : "1",

Then, I'm a bit fed up on straight focus on power usage vs upfront cost here and there. At least on this video, the narrator explains all the concerns regarding power, cost, PSU stress, heat, instability. Well done, again, imho.

Go for the R9-270, yes.

You only mine if/when you want, and you'll learn. If mining it's for you, then invest further on a dedicated rig. If not, enjoy it's capabilities for gaming and multimedia.

Edit: ^ $65 worth of power costs per month, for a machine using 300W?  

I have to say the kits are damn nice for who wants to avoid hassle, noise, heat, power use, space at home, etc... Except for the controller, if a device malfunctions, one loses just 10% or 5% of the total hashrate. If I had them delivered in hand like one can get to a computer store and buy GPUs, I'd buy a controller and a few hashing devices, step by step until the 20.

However the most important metric, $/Kh isn't really that interesting. Even the power savings don't make up for the resale value of the used GPUs.

I just checked and can get 600 Kh/s out of a 7970 with the core at 850 Mhz. That means I'll be able to undervolt from 1.07V to 0.93V or so.

Arbitrary values, but correlated with power usage of the core.
1.07 * 1.07 * 1000 = 1145
0.93 * 0.93 * 850 = 720

40% power savings for 20% loss hashrate, not even talking about power efficiency at lower temperature. In other words, around 180W of the GPU vs 120W of the device, at the wall, per 600 Kh/s.

I still can compete with Scrypt ASICs and can keep mining Protoshares, MemoryCoin, VertCoin and the ASIC resistant coins that will come. Then it will be time for cards with GDDR6

I believe that's enough!

People already suggested cards: R9-270, R9-270X, R9-260, 7850, 7870!

I posted 3 good looking cards. Do you want me to find in pink or purple to match the curtains???

If you want cards that look cute, don't ask here. Google and look at them

Huh? LOL!

LOL! 

It is a tad crazy, yes. The fan cools the core, the rams and the vrms. 

You would need to seriously undervolt and underclock (down to 0.9V or so) and monitor the temperatures. I mean, babysit the cards.

I'm in the same boat. I'm going to need to adapt a cpu fan into a card, cutting wires and using some duct tape or other method.

As usual, promoting low power use and not posting price 

Since you accept escrow and this is not preorder garbage, I'm genuinely interested! No promises whatsoever, though. If I decide to move, I'll post again or send pm

I intend to mine scrypt alts while it's profitable and later on move to help secure Bitcoin network, even at operational loss until the units malfunction.

Thanks. Currently ~$2300 upfront for 3 Mh/s scrypt and 100 Gh/s SHA2.

Can one order a controller and N units?

You will brick it sooner by mining at gamer/overclocker settings. People forget that these are Windows direct3d devices that come with useless (for mining) Crossfire connectors, multiple monitor outputs and Battlefield 4 coupons, ffs!  While gaming, accept shitloads of commands, have pauses, interrupts, perform diverse operations, there's some sort of load balance. In other words, at gaming nothing bangs the card 24/7 on the same transistors and components.

At least, please post Gpu-z screenshot of the "Sensors" tab, a few minutes after start mining ..

Edit: your cards core reached 81 ºC in the winter?? Perhaps the fan was at low speed or the VRM's went above 110ºC