Gregory's Server@marcan that's pretty much what I'd expected at the time, but the problem followed the Ethernet cable. I could repro with nothing else plugged in.
This was (iirc) an Intel L440BX+ motherboard and an early Cisco 4006 switch. I'm pretty confident that whatever was happening, it was *way* out of spec.
Intel's first couple server boards (the T440BX and L440BX+) were *weird*. Like, the T440BX powered the CPU from the PSU's 3.3V line, drawing 2x the amps most early ATX PSUs provided.
@marcan ah, L440GX+, not BX+. That was the one where the AGP bus was bridged into an extra 64-bit PCI bus.