I would concur with the "bad earthing" theory - it could be a well-implemented bad design rather than poor workmanship though, especially in a wooden aeroplane. The switch-mode supplies and the radios need to have their ground-returns "star-pointed" to minimise power-rail-borne interference. It's unfortunately common practice in aircraft installations top "daisy chain" the power cabling; just tapping into the nearest available ground point. This is just about acceptable in a metal airframe because the overall airframe point-to-point impedances are very low, but in a wooden airframe the situation is very different.
Does the ground-return wire from the power supplies go all the way to the main power distribution point (or even the battery bus), or just to a nearby cable for (say) the nav lights?
PDR