4 possibilities
1) the slot is bad.
2) the graphics card is bad. The middle slot is only x8, so maybe x8 of the graphics card is bad.
3) the drivers are bad.
4) overclock is bad.
For 1 and 2, try a different graphics card in each slot. For 3, try a program like Driver Genius Professional or Driver Support to update your drivers. For 4, try non-overclocked speeds for CPU and RAM.
I have the same motherboard and CPU with a Titan X in the first x16 slot (closest to CPU) working fine.