[GreggWardNZ]

Just wondering, was the Arduino Nano soldered directly onto the circuit board, or was an IC holder used first with the Arduino Nano mounted afterwards?

If soldered directly onto the circuit board, could excess heat maybe have damaged some of the sensitive components on the Arduino Nano resulting in this kind of issue?

I made up a socket connection for the Nano, using female header connectors and have always removed the Nano when soldering/re-soldering.

Today I bought a shielded USB cable that advertises that it protects against potential electromagnetic interference, but that made no difference to the problem. After connecting up a USB mains adapter/charger, the Rx board appeared to start working properly. However, after about 5min I swapped the USB cable back into the PC and it continued to work, so I don't really know whether that actually proves anything.

I'm starting to wonder whether the DCCduino Nano (cheap clone) on the Rx board may be faulty. Maybe it could be faulty around the switching circuitry that Werk_AG outlined. I am just in the process of purchasing a genuine Arduino Nano, to see if that will make a difference.