So if you want the $100 test to eliminate PoE electrical effects, get
a pair of media converters and run fiber between them. Put the CPE on
the far end. Optimize as appropriate if you have SFP-capable switches.
Sure--that would shoot down the "leaking non-existent PoE across a motherboard and out another NIC" theory, but I was more thinking along the lines of something like PoE causing RF interference or something.
I mean it's DC not AC so...it wouldn't be putting out a modulating signal that interferes? ...honestly that's outside my knowledge domain.
-A