..even something like the long reach piezo electric gaslighters can be a very handy test source for this kind of fault. Especially if you have the balls to ground one side to your supply gnd. You got a short, shielded arc, which emits HF very strongly and reasonably directionally (use short braids for the ground). Scary, but it works for me, to the point I an dance the arcs anywhere over my boards and they don't glitch.

Steve