Liam,

I just solved my similar problem, but had to change the engine to do it. It is a long tale and I will not hijack your thread by going through it. The suggestion to check ignition ground is only good if you have an electronic ignition. A mag coil ignition does not have such ground. At least my Quadras don't.

I had run the check suggested above and determined that the interference was coming in through the aileron leads. I already had noise traps on them, but I guess the rf was just too much. I replaced the wing extensions with shielded leads to no benefit. In the end the engine change was the solution. Though the problem entry was the ailerons. The problem was coming from elsewhere.

Good luck,

Bedford