Its the heat.. and I did extensive testing to include top and bottom only vs two cards next to each other.

Thanks!

I'm guessing its the temps.. that makes a lot of sense.

Thanks for the massive replies. I'll write back with the results from my testing.

Yep.. I turned off crossfire. I've adjusted the memory.. has no real effect other than heat etc. Yes, I'm using one miner per card. The best set up so far after days of testing is -w 128 -v2 -f2 or -f60(for desktop use).

My setup:

1. Daily use machine

1x6950 reference card with arctic cooling cooler stable @ 950/1375
Windows 7 x64
Consistent 400mhash rate - all the time, 24/7

2. Purpose built miner.
4xMSI 6950's @ 840/1250
1200Watt Antec - Huge Lian Li case etc
Varying Operating systems Vista/Linux

Consistent 1000mhash - all the time, 24/7 Operating System makes little difference if any

Problem: 400mhash from 1 card vs 1000mhash from 4 cards together!!

I've tried enabling crossfire, disabling it, removing the crossfire bridges etc. I've tried Linux, Windows etc!!

Here's the real issue - the numbers seem to go down as I enable the additional cards in the 4x machine. The first card will run at 300-350 out of the gate and settle just above 300mhashes. As I enable each additional card with a new instance of Diablominer or poclbm the overall numbers go down. I've tried everything but manipulating hardware settings on my motherboard which is an Asus Rampage Extreme III (love it). I just don't get it.. I should be getting 1200mhashes at the very least from the 4x miner after things even out. When I first kick off the cards in either crossfire or individually they hit a total of ~1450mhashes and then evenly drift off to ~1000 in crossfire or ~250 per card in non-crossfire mode.

Any ideas? 4 cards at 1000mhashes is pathetic vs 1 card at 400. These are all 6950's at the end of the day and yes, the single card is at 950mhz vs. 840mhz but that's not enough to cut the performance down that far. Remember  we're talking 4x6950's doing a total of ~1000 while one of them is doing ~400.

Please throw me some ideas. I'm using api.bitcoin.cz but I don't think my 5x 6950's are overwhelming his setup.

EDIT: I thought I would mention that the cards are not running at 100% when I have all the cards going at one time which is  a real concern - crossfire or not. This makes me thing one of two things. Motherboard configuration (maybe) or too much going to bitcoin.cz at one time.