6870 Crossfire PSU problems

Hello all knowing tom's hardware community!
Recently I upgraded 3 parts of my computer, these were the power supply, motherboard and a second graphics card (for 6870 crossfire)

The parts are:
AsRock Extreme4 970 Motherboard
Gigabyte 6870 FactoryOC'd to 915mhz (to be coupled with XFX 6870)
and the 750w OCZ ZT power supply

The first problem is, when I use the factory overclocked graphics card in the main pcie slot and crossfire the computer simply won't start, however when I use the non-OCd card in crossfire, or either of the cards on their own it will start up fine.
Secondly, with the working crossfire configuration, 3-5 minutes into high graphic gameplay (BF3) the computer will restart, no beeps sounds or screens.
This has led me to believe the PSU might not have enough grunt? Or maybe the 12v rail overloaded? (62 amps)

Any help would be appreciated, thanks again.

EDIT: Other specs are
AMD Phenom II x4 955 Black Edition
1 tb Western Digital HDD
Standard CD/DVD Drive (don't expect that to be of much importance)
8 gb of DDR3 ram at 1333mhz (2x 4gb)
Also being a Gigabyte card the newer 6870 has 3 fans.
5 answers Last reply
More about 6870 crossfire problems
  1. Two 6870s shouldn't overload a 750W PSU as I would be surprised if such a system drew 500 watts.

    If the PSU is faulty in some way then that could be the cause of the problem.

    When HardOCP tested the 650W version IIRC they failed it because it shut down due to Over Temp Protection and it hadn't exceeded it's operating temperature spec by much if any, which is 45C. So I want to ask what case you use and if you think it's possible that it is being subjected to ambient temperatures above 40C.
  2. I've recently tried launching the computer using ONLY the new card, and within 3 minutes (most often under 1) the computer shuts down. When I use ONLY my old card the computer has none of these problems.

    My case is Antec Three Hundred and the room is around 20-25 degrees Celsius most days. The card that is crashing barely reaches 40 degrees itself which leads me to believe the new graphics card is faulty. What are your thoughts?
  3. I agree that it is sounds like the new graphics card is faulty.
  4. Had the graphics card tested and it all seams to be working.
    Taking it down to a shop to have them "repair" it.
    Only thing I can blame it on is the PSU now. We'll see.
  5. After taking it down to a local computer store and having them assess/fix the computer, it still isn't working. Even after they fixed a "short circuit" and the overheating cpu.
    I've completely ran out of ideas, could it be the PSU? That's my only answer left...
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