Are variations in GPU's normal or do I have a dud?

[mpj76a]

Hello,

I built my first rig.  It consists of two xfx 7950's.  I've been using GUI Miner and MSI afterburner for tweaking.  I've been able to get a total of 1100 M/hash/s out of the system, but it gets pretty warm and unstable at that point.  I'm happy overall since this was about expected from the wikipedia chart.

However, one card will do no more than 530, the other card cranks out 570.  No matter what settings I have, even when idle, one card runs about 5 degrees hotter than the other.  The hotter of the cards crashes much earlier in my tweaking aggression than the cooler card.  They're mounted directly into an ASUS mobo inside a coolermaster LAN box.  I thought that maybe heat accumulation with both cards running and their exact spot in the chassis may be the factor so I ran each card by itself.  No difference.

Is this sort of variation normal?

If I decided to add another card, I'm assuming I would have to locate it outside the LAN box with one of the PCI extender cables I see, correct?

Thanks!

[HellDiverUK]

Not every chip is identical.  Maybe one's heat sink isn't as well attached?  Or the cooling for one card isn't as good in the case?

Have you tried swapping the cards round if they're both in the same machine?