Why wont my 5750/5850s work together, only separate ?

[MacT]

  My two cards are detected by catalyst and guiminer, but wont both work at once.

  I have 5850 and 5750 cards. Extend desktop is selected and a dummy plug is in the 5750. Catalyst reports the 5850 as being the connected monitor, and the 5750 as being " Default VGA monitor [ Single ]". Guiminer sees the 5850 as 0-0 Cypress, and 5750 as 0-1 Juniper. These work fine mining and at their expected speeds if only one is used.

  However, if I try running both at once, to different pools, whichever card started first begins to choke and falls to almost no speed at all. These are run in xp sp3. I have clean installed the newest catalyst multiple times, tried with and without the dummy plug, have plenty of juice on the psu, and have set them to different core affinity. So far nothing gets them both working at once.

  Can anyone make sense of this for me, or better yet, offer a solution to using them together at the same time ?
Thanks- MacT

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[The LT]

Could be a motherboard issue, better specify the full system spec.

[MacT]

Could be a motherboard issue, better specify the full system spec.

 Ah, okay. I had not thought of that.  This is built on a M4A78 PLUS, with AMD 945 Phenom 2 and 3g of ram. Device manager shows both cards, one on pci slot 4, the other at pci slot 2.

[MacT]

 just finished going through every motherboard and catalyst issue out there, still cant get these two cards to mine at the same time. I am out of ideas of things to look at that maybe causing this

[Bobcat]

just finished going through every motherboard and catalyst issue out there, still cant get these two cards to mine at the same time. I am out of ideas of things to look at that maybe causing this

look at this thread,

https://bitcointalk.org/index.php?topic=37911.0