I have a Mac Pro 3,1 w an nVidia 8800 GT running El Capitan. When I reboot, the monitor shows the apple logo with the progress bar. It then goes blank and the monitor reports an out of range error. If I plug another monitor or the same monitor in to the second DVI port without the other monitor being on, the monitor in question displays the login screen without issue.
I'm confused as to what is going on as when I shutdown the monitor is fine. Any thoughts as to what is being triggered when I plug in the cable to the second DVI-D port?