One of our initial tests was to disconnect the coax to the ADF sense antenna, but we never removed the whole antenna, now I am starting to wonder if the idea of that antenna vibrating may have some merit. So far this week the new antenna on the underneath has been perfect, whilst the top antenna still suffers from the same problem.
Next week I will try completely removing the ADF sense antenna to see if that changes anything.
Thanks again for the suggestions guys.