One 7950 card has low KH/s when more cards are present

I have a weird problem (at least for me). I have 3 HD7950 cards in 1 rig (2 Sapphire Dual X and 1 Asus DCU2). When I run the 2 Sapphires alone, they both get over 600KH/s. When I add the Asus to the mix, one of them drops to about 400KH/s and I cannot get it above that. I tried changing clocks,voltages,TC, nothing works. I swapped PCI-X slots, risers, same effect.

It seemed that it's the same card that's slower, until I added an old 5770 I had around (for an extra 200KH/s). Now the other Sapphire is slower. If I run the Sapphire alone and the other two cards are connected (the other Sapphire and the Asus), it won't get past 400KH/s. But if I disconnect the Asus and restart, it will go over 600KH/s.

Does anyone have an Idea about why this is happening? The PSU is a 1200W Corsair, so that can't be the issue.