Howdy all, figured this is probably a good place to start...
I've just setup a rig with 2 x ATI HD 7870, running on Windows Server 2008 R2 and got everything configured (ATI Catalyst / OpenCL drivers, etc).
When running guiminer for some reason the first GPU runs at a decent speed of some 400MH/s, but the second GPU runs at a mere 38MH/s. At some point in time during one of the re-installs I think I did something and caused it to function properly and was doing some 800MH/s in total - only to find out the next morning that the 2nd GPU's gone back down to slug speed. Maybe I was dreaming?
Anyhow, tried BAMT, and got some issues there but won't go into details here and will just focus on windows for now. Tried Windows 7 x64 and had the same result. Rather mind boggling and wouldn't mind ripping my hair out but I'd start to look older than the colleagues at work...
Now, I don't believe there's any hardware faults. I've tried to mine with only the 2nd GPU plugged in (same slot) and on it's own it works beautifully. Got a decent power supply and the rig does stuff all apart from just running a tiny SSD - system runs at some <200 watts.
I've tried both guiminer and even just poclbm on it's own with GPU 2 only and had same result there.
I did have the crossfire cable installed previously but have taken it out after what I read about ppl's comment saying it's best to be left out.
Any help will be very appreciated =) Before I lose any hair I suppose.. heh.