Ascend Charlie
If it is the EFIS you need to put slugs on the power supply cables. I actually doubt it is the EFIS though, I think they are just a symptom.
Radio Frequencies mix up via a thing called intermodulation. This happens in the front end of the radio receiver. For example if you have a 104 MHz FM station nearby and a 24 MHz transmitter of any type close by the FIRST ORDER intermod is 128 MHz. (This is a very simplistic example and is most probably NOT what is happening to you).
A similar thing hapens in Canberra with a 104.7 MHz FM transmitter, the VOR and the 124.5 MHz approach. This is a much more complex third order intermod, but with old radios, all you can hear when flying over the VOR is 'benale' FM radio music.
Airservices know of this, I am not sure of the cost argument though, I personally think their design area can't cope with the homework and maths needed to avoid this sort of problem.
PM me, I'll have a look around Scone and run the numbers for you, do this BEFORE you spend $$$$$$ getting a radio LAME to fix what (s)he won't understand anyway.