Need help with a Shut Down problem?

Hardware Hub GPU
1

Right, so about a week ago I installed a 2nd HD 6870 and ever since then whenever i play battlefield 3 or another demanding game my computer just shuts down for no apparent reason... I thought it was an overheating problem so i instaled another fan and the GPU's run around 50 - 70 c when gaming and i just can't figure it out :'( any advice will be much appriciated :) 

Specs off the top of my head

GPU I5 2500k default clock speed

An asus HD6870 and an XFX HD6870 running in crossfir

Antec One

750w PSU 

First time on the forum so don't shout if i did something wrong!

2

Did you install the 2nd GPU the corect way? Are all of the cords where they should be?

 

3

Yeah, everything is as it shoud be

4

What is the exact model of your PSU? or is it a generic 750W?

Edit: Upon some research into Antec 750W PSU's, there seems to be a trend of faulty 12V rails that 'reboot' your system consistently. If you do have this PSU, it seems to be a design flaw and there isn't much you can do about that...  
Source: http://www.hardwaresecrets.com/article/Antec-High-Current-Pro-750-W-Power-Supply-Review/1632/11 (First 2 Samples faulty/with this issue) 

5

Thank for the reply and it's not antec, it's corsair if that's any help

6

The things that instantly come to mind are that the actual communication of those two cards may be incompatible for an unusual reason (Regarding software I suppose) 
or
They pull too many amps on the PSU at load. What exact model is the PSU? TX? AX? etc... 

7

No idea :/ just says corsair... 

8

Have a look on google images and see what it looks like then post it? 
If you can get a hold of another quality power supply and see if that works for you at load, then it might be the Crossfire or the cards...