I've had the same problem for ages, I found a solution in passing that worked for me:
Do you have any other pci slot devices? If so, remove them and leave just your graphics cards and try and see if it fixes it.
A little info based on my hardware situation:
My motherboard has 5 PCI/E slots, graphics cards are positioned in slots PCIE2 and PCIE4 which run at 3.0 and 2.0 x16 by default, BUT if something is plugged into PCIE1, PCI3, PCI5 the PCIE4 slot drops to x2. It appears latest ATI drivers do not support x16 and x2 crossfiring.