I know you said you tried moving the cards around but make sure that you install each one on a 16x slot first before moving it to a 1x otherwise the driver may not install properly. Also make sure crossfire isn't enabled. You might also need to put a dummy plug on that card to simulate a monitor or even just plug a monitor into it.

I had a 6950 on a 1x slot that would only mine at 1/4 of what it normally would do on a x16 slot, as soon as I plugged the monitor in it went back up to where it should be.