Hi there,
Check the PCI-e settings for PCIe_2 and PCIe_3 in the P6T BIOS. The default is 8x8, if I remember correctly. You should change that to 16x1 for PCIe_2 and PCIe_3.
I had to change that BIOS setting to get my 285GTX to run properly out of PCIe_2 because I have a SB X-Fi Elite...