Hi guys,
Have you run Expansion Slot Utility to make sure the card is getting enough speed through the PCI-E Lanes.
Macintosh HDD>System>Library>Core Services>Expansion Slot utility
Make sure the LHe is in slot 3 or 4 (counting from the bottom up) and give it 4x speed in blue. Ignore the yellow on the diagrams. It is VERY IMPORTANT that you video card (slot1) continues to receive 16x in blue as well.