=Why on earth would an extension cable stop only the desktop from booting when used in card slot number 1?
Not sure if this is your issue or not, but if by saying card slot #1 you actually mean the first card slot (number zero) then the bios might be misidentifying the card as a bootable network card, waiting for it to initialize.
Keeping the same slot and switching the one "cable from hell" changes my results, so I doubt this is the case.