Are you sure the devices have Gigabit ports? Most printers, including big multi functions don't. Some of the older printers also don't auto sense well and some times require hard coding the port to 100mb full duplex or half duplex. If your printer truly is 1gb ethernet I'd suggest doing a packet capture and looking for anything that might be causing an issue. You can always call support for help with the packet capture and they look through those things all day long.
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