Ok, time for another update.
Either using the antenna on the bottom of the fuselage or using either of the top antennas with the ADF sense cable antenna removed does not create the problem.
It has been a week now and I can say that with absolute certainty.
The ADF sense antenna is the culprit!
The antenna that was removed does look suspiciously like welding wire to everybody, but it was installed by an approved outfit and is apparantly a supplied cable that comes as part of a kit.
The antenna has been replaced with a stranded copper wire of the type commonly found on HF and ADF sense antennas, and so far so good. I need another week to be certain that the issue is bannished for ever, but I am hopeful.
With any luck my next post will lay this to rest for good.
Thanks again to everyone for the help and suggestions.