Total posts : 45366
Let’s not forget ground conductivity. Even at these low power levels the quality of ground conductivity can have either positive or negative results. Maybe isolated ground rods seperate from the mains would make a bigger difference than just one ground rod or the grounding in the home wiring. Also there is a remote chance that the ATU has a problem. I have never had one but i assume it has screws and maybe you can check over all the contacts and solder joints just to be sure they are solid.
mram1500 mentioned he has a schematic which could be very helpful as a reference when checking for loose connections, bad solder joints. Good Luck