One test you may still do, is to activate battery test of the ignition. Then check for servo jitter when the ignition is switched on. (the ignition checks battery under ignition firing condition)
If no servo jitter occurs, it it not the ignition that causes this, and you need to look for metal to metal contacts in your airframe, faulty switches faulty wiring etc. Especially broken electrical wires can drive one crazy, and a sound portion of luck is needed to find them.