Looking at your picture of the power connection to the pci riser it looks like it is branching off of a molex plug that would normally be for hard drives or dvd etc. Those connections are generally at 60w and a high powered graphics card expects to pull 75w from the slot. I think that is one very probable cause of the code also considering a wifi card worked but that is far less power consumption. That is my best guess from hardware side of things. The real can of worms is if it is a driver issue.