I'm no expert on this, but suggest a few other things to try or ask.
Does the link fail if you issue the command but have the transceiver
disabled so that it doesn't transmit? Or if it's already transmitting CW?
If you put the transceiver in transmit mode without using commands over the
CAT5, does the link fail? (I'm not sure if you covered this already; you
said the host PC worked fine, but what about the ethernet link?)
It fails at 5W with a dipole too, right?
On SSB can you force it to fail by whistling/humming?
Could there be a ground loop at the transceiver that causes significant
radiation even with the dummy load?
How are you getting the SSB audio between the shack and house?
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