How come you all seem to depend on changing every component in "most expensive first" order then move on to "pot luck" then?
Cos that's what most FIMs tell you to do. Chuck in a load of black boxes and if that don't fix it, look at the wiring. It's not my fault that some of these black boxes cost more than a bungalow! But usually (and I say usually) they fail more often than A/C wiring.
Pot luck? Nah! It's skill mate!