Do you have dummy plugs or the DVI jumpered with resistors?
Also, check your screen resolution control panel. What is the Multiple displays setting set to?
This. Tho i've not needed them I have my monitor connected to one card, when I tired to run it off the MB's onboard DVI I get the same results as you, meaning I'd need dummy plugs if i want to run this particular rig with display on the onboard, so I just left it plugged in the the AMD card and all cards work fine, with the one running the monitor lagging behind by 4khash or so too the others.