Hmmm. Are we running out of things to try. I had an obscure fault years ago with an ISA card, and the fault lay with one of the tracks on the motherboard being cut, so -5v wasn't going down the bus. I wonder if it's some strange problem like that. Seems strange that you've tried different cards / card slots, and nothing works.
If you like, I don't mind sending you (yet another) LAN card for you to try - an Intel one, which you might have more success with.