Ok, time for another update.
After one week with the antenna on the underside of the fuselage I can say that Com1 is definitely no longer suffering from the same issue.
Com2 on the other hand is unchanged, this proves it is something occurring on the top side of the fuselage.
Thanks for the ADF sense wire suggestions, It has been on the aircraft since new, however the wire was replaced in June last year, the problem started several months later.
The ADF wire will be removed for testing to see whether Com2 also behaves perfectly with the wire removed, another update soon.
Those people who are surprised by the age of the aircraft should have a look at the average age of the GA aircraft fleet and you will see this is nothing out of the ordinary. Charter places very strict requirements on the maintenance of the aircraft and nothing that could affect the safety of the aircraft is overlooked.
I was specifically looking for a 4 place aircraft that could be used for charter and was also fully aerobatic with 2 seats filled, the Fuji was the only one I could find.