That is already half of a solution:
Go for fiber. That is imune to both ground and RF problems. Avoid ground connections between the equipment.
Replace ethernet with fiber. Break serial lines with optical isolators.
Yes, fiber will solve ground loop problems. And this smells like a ground loop issue. But at this juncture, I don't have enough specifics to recommend ma$$ive changes.
Cut the ground wire in your power cords but ground the equipment directly to a metal frame.
I would NEVER tell a client to do this. That could easily kill someone.
Avoid ground loops: Between two computers you have a ground connection via the powerline ground. Connect them via RS-232 and you have a second connection via the RS-232 ground. If your power ground is bad then you might run amperes through the RS-232 ground that results in Volts, more than your signal level, maybe.
I don't recall mention of RS-232,. but yes, this is the classic example of ground loops. -- A host is a host from coast to coast.................wb8foz@nrk.com & no one will talk to a host that's close........[v].(301) 56-LINUX Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433