Paddybee
Have scanned this thread and thought that maybe what you have is a Radio Symptom as opposed to a Radio Problem- ie the symptom manifests as a radio issue but the condition is totally unrelated. The various cases where the aircraft was used all day and issue was present, only to disappear when taken for ground testing makes me think: "What if this is caused by presence or absence of front seat pax, or caused by another factor in the flight profile that is concomittant to the RPM and not actually the RPM at all? AOA or trim at that weight/speed condition?" It appears that all the logical steps have been taken (for a radio problem) already. Good luck if this starts a new chain of thought.