Turns out some motherboards simply have specific slots that just won't allow "powerful" GPUs to function properly. I'm not a super PC guy that explain things well but here's from tomshardware website: (From what I understand it is a 2.0 slot and there is nothing I can do about it. So as recommended plugged it in the 3RD slot and it works like a charm now.)
The ASUS X99-PRO comes with three PCI Express 3.0 x16 slots (PCIEX16_1, PCIEX16_3, and PCIEX16_4). With a 40-lane processor installed, the board supports x16/x16/x8 configuration. With a 28-lane CPU, the board supports x16/x8/x4 configuration. In both cases, the last slot (PCIEX16_4) shares bandwidh with the on-board M.2 slot; if this M.2 slot is used, this PCI Express 3.0 x16 slot is disabled.
There is also one PCI Express 2.0 x16 slot (PCIEX16_2), which works at x4 speed (x1 speed if the additional USB 3.0 ports are enabled), and two PCI Express 2.0 x1 slots.